It should be easier to diagnose time outs from spooling application to CUPS.

Ran Adobe Lightroom to produce a large print. Due to machine's age, etc., job took a long time (5.5 minutes). CUPS Timeout setting at default is 300 seconds. CUPS error log says

E [17/Apr/2019:21:50:17 -0700] [Job 3] Aborting job because it has no files.
E [17/Apr/2019:21:50:40 -0700] [Client 44] Returning IPP client-error-not-possible for Set-Job-Attributes (ipp://localhost/jobs/3) from localhost.
E [17/Apr/2019:21:57:10 -0700] [Job 4] Aborting job because it has no files.
E [17/Apr/2019:21:57:41 -0700] [Client 59] Returning IPP client-error-not-possible for Set-Job-Attributes (ipp://localhost/jobs/4) from localhost.

which gives no hint that there was a timeout.

Doing this on MacOS, I could use cupsctl to set Loglevel to debug, and then I saw the timeout. Changed Timeout to better value & things work as expected. As a user, I shouldn't have to go that deep to diagnose things. Log level for timeout message should be changed.