The fix for this issue is now available in the latest update of InDesign – that is, InDesign 15.0.2.
Please update your InDesign application to get this fix.
Adobe InDesign team
An error occurred while saving the commentAnonymous commented
I can't share my buggy file due to client confidentiality but this is now happening to me too. The buggy file is a book-length doc with lots of paragraph styles. I updated to 2020 several weeks ago and this is the first file where the bug has shown up. From what I can tell it doesn't revert to the last saved version, but rather to some indeterminate earlier version of the file (always the same point, but I can't tell why it's going back to that particular point). I lose all changes that were made after that point, including new pages, new styles, changes to masters, etc.
I am working on a late 2013 Mac, 10.14.6. This is an incredibly frustrating bug and I hope a fix is released soon.