Thank you for your quick message. We were able to fix the “error” in the meantime, although it was really very strange how Kirby behaved here. I would never have thought that it was related to the upload_tmp_dir that Kirby can no longer write the files.
But thank goodness we found the error after some hours of research. Of course we also tried it with a new StarterKit and got the same errors there.
The hoster has carried out a domain change for us and has probably adapted the upload_tmp_dir to the old domain and forgotten to change it during the domain change.
At first I thought it was because I manually reset a multilingual instance to a single language. Also checked plugins, caching etc. but found no errors. It was just strange that only some fields could be saved and some not. Doesn’t really make sense to me yet either