*Oct 6 16:55:17.307: %CAPWAP-3-ERRORLOG: Retransmission count for packet exceeded max(UNKNOWN_MESSAGE_TYPE (5)
*Oct 6 16:55:17.307: %CAPWAP-3-ERRORLOG: GOING BACK TO DISCOVER MODE
*Oct 6 16:55:17.307: %DTLS-5-SEND_ALERT: Send FATAL : Close notify Alert to 172.16.0.40:5246
Timeout is happening as it is not able to reach the WLC. Is the WLC pingable for a long duration from the AP ? If yes , it could be a WAN issue also. It should be analyzed properly using a TAC case.
You will need WLC side logs and WLC version etc to check for bugs etc. If you have the liberty and you want to rule out WAN issue , you can create a GRE tunnel from the AP location to WLC location and route AP/WLC traffic via it and see if that resolves the issue.
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
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