AWS_FOTA: ERROR: mqtt_connect -22

Hello, I've been having problems getting the AWS_FOTA sample working on a nrf9160 DK.
At first, the device got stuck as it was downloading the firmware update, at 2%. I was able to fix this by setting CONFIG_DFU_TARGET_MCUBOOT_SAVE_PROGRESS=n. After that, the download got stuck at around 11%, though that number changed a bit each time I ran it. While I was trying to fix that, the device stopped being able to connect to AWS at all, even though I didn't change anything. The device waits for a long time after the "client_id:" line, and then seems to time out and print "ERROR: mqtt_connect -22". The timeout/error seems to come from mqtt_connect -> mqtt_client_tls_connect -> connect (line 87 in mqtt_transport_socket_tls.c). I am able to connect to AWS IoT from a python client on my computer using the same credentials and MQTT broker, so I don't think it's a problem with the AWS server.
Here are the changes I made to prj.conf:
CONFIG_DFU_TARGET_MCUBOOT_SAVE_PROGRESS=n
CONFIG_CLOUD_CERT_SEC_TAG=5
CONFIG_APP_VERSION="v1"
CONFIG_MQTT_BROKER_HOSTNAME="ahykgwaormc93-ats.iot.us-west-2.amazonaws.com"
CONFIG_USE_CLOUD_CLIENT_ID=y
CONFIG_CLOUD_CLIENT_ID="nrf9160dk"
CONFIG_USE_NRF_CLOUD=n
Besides that and the credentials (ran once with CONFIG_PROVISION_CERTIFICATES), I didn't make any changes to the AWS_FOTA sample.
I tried both the master branch and the v1.2.0 branch. I also stuff like turning CONFIG_DFU_TARGET_MCUBOOT_SAVE_PROGRESS on again, and re-flashing the credentials with CONFIG_PROVISION_CERTIFICATES again.
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