Kein Export in LR möglich seit Update

Try resetting LR's preferences:
https://www.lightroomqueen.com/articles-page/how-do-i-reset-lightrooms-preferences/

LR sometimes soils its preferences file, and resetting it can fix all sorts of wonky behavior. That article explains how to restore the old preferences if resetting doesn't help.

Someone else recently experienced the same error message, and resetting preferences worked for him:

https://community.adobe.com/t5/lightroom-classic-discussions/some-export-operations-not-performed-an-unknown-error-occurred/td-p/12916323

Starting today, xmp files are not updated when I make edits in Develop.  Changes continue to show up in the History pane. Automatic save to xmp is checked. Activity Center has Saving xmp unpaused.  When I save manually, I get message "one or more of these photos has been changed in an external application," but these photos have never been edited in any other application.

Any suggestions for fix are welcome!