Fix is available in the latest update of InDesign.
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Adobe InDesign team
An error occurred while saving the commentAnonymous commented
I could not get anything but a black screen (and apparently an invisible dialog or alert).
I use a MacBook Pro, MacOSX 10.16.7 ("Catalina").
Start InDesign, go to Window menu, choose Character (now, some things begin to appear on screen)
In upper right, change from Essentials to Printing and Proofing.
Now I can create a new document or edit an existing one!
A few details: If I tried anything but Window menu, I could not get InDesign to quit; I got
"Cannot handle the request because a modal dialog or alert is active".
Going into Library Preferences and Cache and clearing InDesign stuff gave no relief.
Doing Uninstall then Reinstall of InDesign 2021 did not appear to fix anything.