cannot add metadata to old pics

Using Bridge 2021 on a Mac with BigSur 11.2.3:

It runs fine with actual files, but when I want to add a keyword to an old pic (which got its keyword with Bridge CS4 odr CS6) I get the following message: While writing of metadata on file xxx an error occured.
Reinstalling of Bridge didn't help. Hopefully somebody knows how to solve that.