When adding a cover picture to multiple files either through the "Tag Quellen"->"Cover Art" or by copying it from the clipboard to the cover field, some files will get randomly corrupted. In that case I get a message that says that file xxx.flac could not be opened for writing. The corresponding file does still exist, but with a temporary file name.
Renaming the temporary file to the original name fixes the problem, so the file itself doesn't seem corrupted.
I could see this behaviour in both v2.65 and v2.70. I have never tried to add cover pictures with older versions.