I had a similar problem with unknown timing – It was a file stuck in the Batch folder. I couldn’t even erase it. Even turning off the whole computer did not remove the file from the Batch folder. Here’s a solution I found on the Apple support site…and it works.
1. In Finder select go, go to folder
2. Enter: var/spool/qmaster
3. This will find a folder (oddly named with numeral & text). Trash the folders you see INside this folder (mine were called Shared & Jobs
4. In compressor, select the menu item Compressor/reset Background Processing.
John.