Dear Kyle,
thank you for the assistance,
I have now found the problem and solved it.
I'd like to share following with other users that might have the same problem:
1) The problem occured in my case because I had updated CB as well as some plugins, but some other plugins were inconsistent after the update, which I didn't realise at that time.
2) The CB Tool to test the Plugin Database helped me to find the inconsistent plugins.
3) I reinstalled all inconsistent plugins
4) some old plugins where not anymore useful, functions were taken over by other plugins. I deleted these plugins.
5) At the end, all lines in the tool test were green.
6) after having done this, the field editing function worked again.
Best regards
Peter