Just for the awesome plesk-devs that doesn't seem to know that. If you want to tar a file with a starting hyphen / dash you use -- to seperate options from parameters. Like:
tar -czf backup.tar.gz -- -fileStartingWithDash.php subfolder/-anotherFile.js
Had the same problem.
The file /opt/psa/admin/htdocs/images/custom_buttons/-cbOKoQ was causing it in my case. It's a png image. I attached it here. Seems like it is from the uptime-Robot-Extension.
I never placed it there and I think official Plesk plugins ( Uptime Robot extension - Plesk )...
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