Unfortunately, your error_log files do not contain the relevant information as for your CUPS the CUPS-internal default log rotating seems to be active. This restricts logs to a size of 1 MB discarding the needed information within minutes. Your logs cover only ~30 min.
So run
cupsctl MaxLogSize=0
and do all your tests again.
Note that MaxLogSize=0 is a Ubuntu default which we introduced as we did not get useful error_log files on bug reports. Ubuntu uses the central logrotate facility which restarts logging once in 24 hours and archives the logs of the last 7 days.
Unfortunately, your error_log files do not contain the relevant information as for your CUPS the CUPS-internal default log rotating seems to be active. This restricts logs to a size of 1 MB discarding the needed information within minutes. Your logs cover only ~30 min.
So run
cupsctl MaxLogSize=0
and do all your tests again.
Note that MaxLogSize=0 is a Ubuntu default which we introduced as we did not get useful error_log files on bug reports. Ubuntu uses the central logrotate facility which restarts logging once in 24 hours and archives the logs of the last 7 days.