The dreaded File xxx cannot be opened for writing. error

I find this problem has been discussed here for more than a decade. Lots of suggestions tossed out. No files or folders are read only here. I find a suggestion on ransom ware detection, but I wasn't aware of such a thing on Windows 10, and besides, I use Malwarebytes Pro.

All of these files that Mp3tag won't edit can be edited with no problem in VLC and Tag&Rename. But those programs can't remove junky tags and in my case right now, extraneous art. So I'm stuck.

It's happening often enough I that I now dread clicking the Save button. And when it errors, it errors on every FLAC file in the folder.

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.

But VLC and Tag&Rename edit the tags just fine. All those non-standard ID3 tags are long gone.

If you are happy with those programs - fine.
If you want to use MP3tag and you can't and all possible causes like access rights and simultaneous access are ruled out, then the only hint I can give: check the files for corruption.
Or create them anew and see whether that helps.
Unless you have ways to analyze the root cause, then the only recipe remains "trial and error". Theoretical discussions will not change anything.

There must be some corruption in the tags. I took one of the problematic folders. In Tag&Rename I added a cover. This now made three, but Tag&Rename isn't smart enough to know this.

I went back to Mp3tag. It could now read the files and remove the extraneous art.

Even simpler. I don't have to make any changes. Just re-saving the tags with Tag&Rename works to make the file okay. VLC does also, but only for one track at a time.

So the "MP3tag only reports that what the OS reports back." makes no sense for these files.

I'm closing this here, follow-up discussion happens here: