Shadow delta CoAP request failing with 4.06 Not Acceptable despite stable setup (nRF9151)

[Akshay]

I need a immediate response on this. 

I am facing a persistent issue with shadow delta requests on my nRF9151-based application using nRF Cloud over CoAP since this morning.

Setup details:

• Device: nRF9151 custom board

• SDK Version: 2.7.0

• Modem Firmware: 2.0,1

• Protocol: NBIoT with CoAP

• SIM: Jio(India)

My application was working fine until recently, with no firmware or configuration changes.

Observed behavior:

• Shadow delta request triggers +CSCON transition to connected mode.

• Immediately gets CoAP response code 4.06 (Not Acceptable).

• Error log:

<err> nrf_cloud_coap: Unexpected result code: 4.06 
<err> shadow_support_coap: Failed to request shadow delta: 134

Network attachment and registration are stable:

AT+CGATT?
+CGATT: 1
AT+CEREG?
+CEREG: 5,5,"C014","0000C092",9,,,"00001010","00000011"

The issue is only with shadow delta requests. Other nRF Cloud operations remain unaffected.

Troubleshooting performed so far:

1. Deleted and re-added the device on nRF Cloud → temporarily works, but issue returns after subsequent shadow delta requests.

2. Confirmed no recent firmware changes.

3. Verified SIM and network connectivity are stable, data is flowing, RRC state transitions are normal.

Questions:

1. What could cause CoAP 4.06 Not Acceptable specifically for shadow delta requests while all other operations remain functional?

2. Has there been any recent change to nRF Cloud CoAP shadow service regarding Accept or Content-Format options?

3. Could an invalid shadow document cause this persistent rejection? If so, what is the recommended approach to sanitise or reset shadow state without deleting and re-adding the device?

[Akshay]

This is my configuration 

Full log 

[00:11:01.281,158] <err> net_coap: Timeout in poll, retrying send
[00:11:01.739,288] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:11:01.739,318] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:11:11.739,440] <inf> shadow_support_coap: Checking for shadow delta...
[00:11:11.740,936] <err> net_coap: Unexpected ACK or Reset
[00:11:11.740,966] <err> net_coap: Error handling response
[00:11:14.741,699] <err> net_coap: Timeout in poll, retrying send
[00:11:14.764,770] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:11:14.764,770] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:11:24.764,923] <inf> shadow_support_coap: Checking for shadow delta...
[00:11:24.766,418] <err> net_coap: Unexpected ACK or Reset
[00:11:24.766,448] <err> net_coap: Error handling response
[00:11:27.767,181] <err> net_coap: Timeout in poll, retrying send
[00:11:27.820,220] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:11:27.820,251] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:11:37.820,404] <inf> shadow_support_coap: Checking for shadow delta...
[00:11:37.821,899] <err> net_coap: Unexpected ACK or Reset
[00:11:37.821,929] <err> net_coap: Error handling response
[00:11:40.322,540] <err> net_coap: Timeout in poll, retrying send
[00:11:40.854,736] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:11:40.854,736] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:11:50.854,888] <inf> shadow_support_coap: Checking for shadow delta...
[00:11:50.856,384] <err> net_coap: Unexpected ACK or Reset
[00:11:50.856,414] <err> net_coap: Error handling response
[00:11:53.357,025] <err> net_coap: Timeout in poll, retrying send
[00:11:53.966,186] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:11:53.966,186] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:12:03.966,339] <inf> shadow_support_coap: Checking for shadow delta...
[00:12:03.967,834] <err> net_coap: Unexpected ACK or Reset
[00:12:03.967,864] <err> net_coap: Error handling response
[00:12:06.468,475] <err> net_coap: Timeout in poll, retrying send
[00:12:07.021,667] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:12:07.021,667] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:12:17.021,820] <inf> shadow_support_coap: Checking for shadow delta...
[00:12:17.023,315] <err> net_coap: Unexpected ACK or Reset
[00:12:17.023,315] <err> net_coap: Error handling response
[00:12:17.767,028] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:12:17.767,028] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:12:27.767,181] <inf> shadow_support_coap: Checking for shadow delta...
[00:12:30.769,165] <err> net_coap: Timeout in poll, retrying send
[00:12:30.862,457] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:12:30.862,487] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:12:40.862,609] <inf> shadow_support_coap: Checking for shadow delta...
[00:12:40.864,105] <err> net_coap: Unexpected ACK or Reset
[00:12:40.864,135] <err> net_coap: Error handling response
[00:12:43.364,746] <err> net_coap: Timeout in poll, retrying send
[00:12:43.856,994] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:12:43.856,994] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:12:53.857,147] <inf> shadow_support_coap: Checking for shadow delta...
[00:12:53.858,642] <err> net_coap: Unexpected ACK or Reset
[00:12:53.858,673] <err> net_coap: Error handling response
[00:12:56.359,283] <err> net_coap: Timeout in poll, retrying send
[00:12:56.783,447] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:12:56.783,477] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:13:06.783,599] <inf> shadow_support_coap: Checking for shadow delta...
[00:13:06.785,095] <err> net_coap: Unexpected ACK or Reset
[00:13:06.785,125] <err> net_coap: Error handling response
[00:13:09.285,736] <err> net_coap: Timeout in poll, retrying send
[00:13:09.777,923] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:13:09.777,923] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:13:19.778,076] <inf> shadow_support_coap: Checking for shadow delta...
[00:13:19.779,571] <err> net_coap: Unexpected ACK or Reset
[00:13:19.779,602] <err> net_coap: Error handling response
[00:13:22.704,345] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:13:22.704,406] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:13:32.704,528] <inf> shadow_support_coap: Checking for shadow delta...
[00:13:35.206,390] <err> net_coap: Timeout in poll, retrying send
[00:13:35.698,852] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:13:35.698,883] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:13:45.699,005] <inf> shadow_support_coap: Checking for shadow delta...
[00:13:45.700,500] <err> net_coap: Unexpected ACK or Reset
[00:13:45.700,531] <err> net_coap: Error handling response
[00:13:48.201,141] <err> net_coap: Timeout in poll, retrying send
[00:13:48.625,335] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:13:48.625,335] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:13:58.625,488] <inf> shadow_support_coap: Checking for shadow delta...
[00:13:58.626,983] <err> net_coap: Unexpected ACK or Reset
[00:13:58.627,014] <err> net_coap: Error handling response
[00:14:01.127,624] <err> net_coap: Timeout in poll, retrying send
[00:14:02.130,798] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:14:02.130,828] <err> shadow_support_coap: Failed to request shadow delta: 134
+CSCON: 0
+CEREG: 5,"C014","0000C090",9,,,"00001010","11000001"
*** Booting MCUboot v2.1.0-dev-daf2946a0f07 ***
*** Using nRF Connect SDK v2.7.0-5cb85570ca43 ***
*** Using Zephyr OS v3.6.99-100befc70c74 ***
I: Starting bootloader
I: Primary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
I: Secondary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
I: Boot source: none
I: Image index: 0, Swap type: none
I: Bootloader chainload address offset: 0x10000
*** Booting nRF Connect SDK v2.7.0-5cb85570ca43 ***
*** Using Zephyr OS v3.6.99-100befc70c74 ***
[00:00:00.251,556] <inf> main: nRF Cloud multi-service sample has started, version: 1.0.0, protocol: CoAP
[00:00:00.251,739] <inf> cloud_connection: Enabling connectivity...
[00:00:00.530,548] <inf> cloud_connection: Setting up nRF Cloud library...
[00:00:00.532,043] <inf> cloud_connection: Waiting for network ready...
+CGEV: EXCE STATUS 0
+CEREG: 2,"C013","0000BD90",9
+CSCON: 1
+CSCON: 0
+CEREG: 2,"C014","0000C092",9
+CSCON: 1
+CGEV: ME PDN ACT 0,1
[00:00:53.371,154] <wrn> nrf_modem_lib_netif: No IPv4 address given by the network
+CNEC_ESM: 51,0
+CEREG: 5,"C014","0000C092",9,,,"00001010","11000001"
%XTIME: "22","52607280521322","00"
+CGEV: IPV6 0
[00:00:53.373,687] <inf> cloud_connection: Network connectivity gained!
[00:00:54.373,809] <inf> cloud_connection: Network is ready
[00:00:54.373,840] <inf> cloud_connection: Connecting to nRF Cloud
[00:00:54.374,450] <inf> cloud_connection: Device ID: nrf-359404230002876
[00:00:55.009,246] <inf> dtls: IMEI:                  359404230002876
[00:00:55.009,277] <inf> dtls: Modem FW version:      mfw_nrf91x1_2.0.1
[00:01:00.015,869] <inf> nrf_cloud_coap_transport: Request authorization with JWT
[00:01:01.183,074] <inf> nrf_cloud_coap_transport: Authorization result_code: 2.01
[00:01:01.183,166] <inf> nrf_cloud_coap_transport: Authorized
[00:01:01.183,563] <inf> nrf_cloud_coap_transport: DTLS CID is active
[00:01:02.442,260] <inf> cloud_connection: Connected to nRF Cloud
[00:01:02.442,413] <inf> shadow_support_coap: Checking for shadow delta...
[00:01:02.443,634] <inf> nrf_cloud_fota_poll: Checking for FOTA job...
[00:01:02.872,222] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:01:02.872,222] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:01:03.383,209] <inf> nrf_cloud_coap: No pending FOTA job
[00:01:03.383,239] <inf> nrf_cloud_fota_poll: No pending FOTA job
[00:01:03.863,189] <inf> application: Waiting for modem to determine current date and time
[00:01:03.863,250] <inf> application: Current date and time determined
[00:01:12.872,375] <inf> shadow_support_coap: Checking for shadow delta...
+CSCON: 0
+CSCON: 1
[00:01:15.102,661] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:01:15.102,661] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:01:25.102,813] <inf> shadow_support_coap: Checking for shadow delta...
+CSCON: 0
[00:01:27.604,675] <err> net_coap: Timeout in poll, retrying send
+CSCON: 1
[00:01:28.479,095] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:01:28.479,125] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:01:38.479,278] <inf> shadow_support_coap: Checking for shadow delta...
+CSCON: 0
+CSCON: 1
[00:01:39.904,541] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:01:39.904,571] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:01:49.904,724] <inf> shadow_support_coap: Checking for shadow delta...
+CSCON: 0
+CSCON: 1
[00:01:51.454,956] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:01:51.454,986] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:02:01.455,139] <inf> shadow_support_coap: Checking for shadow delta...
+CSCON: 0
+CSCON: 1
[00:02:02.944,396] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:02:02.944,396] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:02:12.944,549] <inf> shadow_support_coap: Checking for shadow delta...
+CSCON: 0
+CSCON: 1
[00:02:14.491,821] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:02:14.491,851] <err> shadow_support_coap: Failed to request shadow delta: 134
[00:02:24.491,973] <inf> shadow_support_coap: Checking for shadow delta...
+CSCON: 0
+CSCON: 1
[00:02:25.985,229] <err> nrf_cloud_coap: Unexpected result code: 4.06
[00:02:25.985,229] <err> shadow_support_coap: Failed to request shadow delta: 134