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 commentAndrea Barbieri commented
I had the same problem. Very annoying. Then I tried deleting InDesign preferences and relaunching ID, and the problem seems to have gone away:
Solution found at https://community.adobe.com/t5/indesign/sos-indesign-won-t-save-save-as-and-i-can-t-undo/m-p/10866948?page=1