This is the first time I saw this message in the logs, and it does not make sense to me:
Apr 20 14:18:01 fax3 asterisk: DEBUG: pbx_spool.c:523 in scan_service: Filename: /var/spool/asterisk/outgoing/http2fax_5e9d6a5083b61502417300.call, Retries: 0, max: 0 Apr 20 14:18:01 fax3 asterisk: DEBUG: pbx_spool.c:344 in safe_append: Outgoing Local/faxout@fax_caller3: DelayedRetry Apr 20 14:18:01 fax3 asterisk: DEBUG: pbx_spool.c:528 in scan_service: Delaying retry since we're currently running '/var/spool/asterisk/outgoing/http2fax_5e9d6a5083b61502417300.call'
See how the callfile is put in the spool directory by the application, picked up by Asterisk and then it is said that it is already being processed. This error only happened once time after the server performed about 300 calls within the 12 days it is up in this configuration.
More precisely: This call was also repeated (retrial is handled by the application) 10 times, and this error occured all the 10 retries. Other calls which were made between the retrials went through fine, so it affected only this call (the call file is always created with the same name).