The issue has been fixed in the latest release of InDesign – InDesign 2021.
Please update to this new version of InDesign to get the fix.
Adobe InDesign team
An error occurred while saving the commentPřemysl Bukovský commented
With InDesign (ver. 15) I've getting this pop-up: "...idml won't open..." Comp's share with Illustrator and Photoshop works fine.