Fix for this issue is now available in the latest update of InDesign 2021 – that is, InDesign 16.2.1
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 still face this problem.
Adobe InDesign Team
An error occurred while saving the commentB. Eshuijs commented
We got this too and is a serious problem for us! It happens with jpg and png. Not all designs had this while exporting, but i could'nt find a real difference in how it was set up (all designs had psds with transparency and a jpg as a background).