XBee 3 LTE-M/NB-IoT "Unknown Error" 0x85 TX Response

I am using a XB3-C-A2-UT-001 to periodically send requests to a HTTP server. This is working great most of the time, but occasionally after sending the TX Request frame (0x20), the module will send a Transmit Response frame with unknown error (0x85). It appears to have some correlation with the module taking longer to connect to the cellular network, although this is not always the case. I suspect this is caused by spotty cell service, but I want to make sure. Are there any known issues that cause this error to occur? Any insight would be helpful, thanks!

XBee firmware version: 1141
U-Blox firmware version: L0.0.00.00.05.12,A.02.19

I am not aware of any specific reasons to the Error. If I recall, it is a generic error that encompasses several causes.

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I am having this same error. I have a setup with the XBee developement board where it is sent a HTTP GET request API packet over serial every 15 minutes. Most of the time it responds with a TX Response success message, but 2-3 times a day I get a TX Response failure message with unknown error (0x85).

Does anyone have an idea about what causes TX Requests to fail with unknown error 0x85? What are the ‘several causes’ that you are referring to?

An Unknown Error means just that. Unknown. The base cellular module found within the XBee module is reporting an “Unknown Error”. It generally can be anything from a connection issue, to a socket error, to a corrupt TCP/UPD packet. Digi support may be able to find out more if you contact them and provide a Modem log file from the XBee. But it may not provide any additional information.

Thank you for the response. I have contacted Digi Support.