Fix for this issue is now available in the latest update of InDesign 2020 – that is, InDesign 15.0.1
In the Creative Cloud Desktop application, click on the Updates section on the left. You should be able to see the Update available for InDesign. Please update your InDesign application to this version to try out the fix.
Let us know in the comments section if you face any problem with the updation process.
Adobe InDesign Team
An error occurred while saving the commentJLS commented
I have the same problem here. At least it's possible to re-name the old file (I add "delete" at the end) and usually a day or two later it's possible to delete it.
But this is absolutely an issue that needs to be solved.