nrf5340 multi-image DFU update over BLE 'Remote Error: In Value(3)' ?

Hi guys - need your help on this!

nrf5340 based board with no external flash (so using nrf5340 on chip flash only)

using SDK v2.3.0 on MacOS

testing DFU update using nrf Connect Mobile on iOS v2.6.7, Build 34 
(as well as testing in our own iOS app)

The problem:

DFU OTA over BLE update of the Application Image using "app_update.bin" has worked for a long time & we have no problems with it. This works perfectly BOTH in nrf Connect for iOS well as in our own iOS application. Has worked reliably for more than 1 year now. 

However, occasionally we need to do a "simultaneous" multi-image update of BOTH the Application & Network Cores.
We can't get this to work & have tried many different configuration options. We always get the error "'Remote Error: In Value(3)" in nrfConnect Mobile on iOS (as well as in our own iOS application which includes DFU updates).  Have tried it numerous times with AND without the "Erase App Settings" switch set on (or off). The setting of the "Erase App Settings" seems to make no difference on the error returned.

I have read/studied the following:

https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/working_with_nrf/nrf53/nrf5340.html#fota-updates 

and

https://github.com/hellesvik-nordic/samples_for_nrf_connect_sdk/tree/main/bootloader_samples/nrf5340

as well as read all the other related cases on devzone. But somehow I/we can't get it to work in our own firmware builds! So I must be missing or overlooked something fundamental!

Below are some screenshots from nrf Connect Mobile for iOS showing "success" when using "app_update_bin" but failure when using "DFU_application.zip". 
I have also included the log from nrf Connect for iOS when it fails with the multi-image update.

Below are copies of our prj.conf, hci_rpmsg.conf & mcuboot.conf

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#
# Copyright (c) 2021 WearSense LLC
#
# WEARSENSE FIRMWARE REVISION TO BE UPDATED HERE!!!
CONFIG_BT_DIS_FW_REV_STR="09.21"
# CONFIG_WATCHDOG=y
CONFIG_REBOOT=y
# Activate Power Management (reduce power requirements
# see: https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/optimizing-power-on-nrf53-designs
CONFIG_PM=y
CONFIG_PM_DEVICE=y
CONFIG_PM_DEVICE_RUNTIME=y
CONFIG_BOARD_ENABLE_DCDC_APP=y
CONFIG_BOARD_ENABLE_DCDC_NET=y
CONFIG_BOARD_ENABLE_DCDC_HV=y
CONFIG_SERIAL=n
# Errata 160 included in v2.3.0 - ensure System Clock is enabled
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

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#
# Copyright (c) 2021 Nordic Semiconductor ASA
#
# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
#
# required for max power savings!
CONFIG_LOG=n #changed
CONFIG_SERIAL=n
CONFIG_CONSOLE=n
CONFIG_IPC_SERVICE=y
# CONFIG_IPC_SERVICE_BACKEND_RPMSG=y
CONFIG_IPC_SERVICE_BACKEND_RPMSG_WQ_STACK_SIZE=4096
CONFIG_BT_RX_STACK_SIZE=4096
# BLE Settings
CONFIG_BT=y
# CONFIG_BT_SETTINGS=n # for now we take care of our own BLE settings (no pairing, etc).
CONFIG_BT_CENTRAL=n
CONFIG_BT_PERIPHERAL=y
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

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# CONFIG_MCUBOOT_LOG_LEVEL_WRN=y
CONFIG_SERIAL=n
CONFIG_CONSOLE=n
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

 

Would really appreciate if you guys could have a quick look at this & let me know what I am missing!

Thanking you in anticipation

Best Regards

Gerard