I've just identified that Greasemonkey prevents some Google Sheets from opening correctly. They try to load eternally and the tabs can't be displayed.

A blessing for efficiency. Eternally grateful!

good

It's failing a lot, Greasemonkey is "removing" my scripts, and failing to restore the backup.
I had to migrate to Tampermonkey, I'm not having problems with it, but I hope you will identify the problem with Greasemonkey and fix it soon.