The dreaded File xxx cannot be opened for writing. error

How should MP3tag guess that you have even more files in stall that are apparently up the spout?

I would seriously consider that the files have been corrupted. Looking at

which found its root cause in non-standard ID3 tags instead of only vorbis comments, I wonder what could be wrong with the new set of files.
I think that foobar2000 has utilities to repair flac files.