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 commentK. commented
No - this issue is not fixed. I have just experienced this with v16.1
I got around it by making a new document and saving it (any/size/shape/format). Then export the problem document as IDML.
Whilst the problem doc was still open on screen I opened the IDML and checked everything was ok by comparing with the problem document.
I was then able to save the new IDML as standard 2020 Indesign file.