Can't export photos from Lightroom Classic v11 ("The file could not be written")

I am running Lightroom Classic v11.0.1 on a Mac running macOS 12.0.1 and when trying to export photos I get the following message:

"Some export operations were not performed. The file could not be written"

I found several articles about this problem but could not find any solution.

What could cause this problem and how to fix it?

Thanks for your help!