I came across this thread after I encountered a similar problem with a single WAV file. I want to share my experience because in this case I believe the problem didn't have anything to do with Windows security, permissions, etc., but with the integrity of the file.
I was tagging my WAV files collection. After having done over 2000 files successfully with Mp3tag (what a great tool it is!), I came across one WAV file which kept getting the message "File Cannot Be Opened for Writing". The rest of the files in the same folder did not get this message and were tagged just fine.
The problematic file played fine in foobar2000 and could be copied to other disks without any problem (the issue wasn't disk corruption then). When checking its integrity, foobar2000 did not report any problems. However, trying to tag it in Mp3tag always resulted in the same error message.
After trying various things (like resorting to a backup copy of the same file, etc.), I tried something similar to what someone described above: I compressed the file to FLAC and then decompressed it. This time Mp3tag created a tag for it (for the decompressed file) without any problems.
Now, when I compared the original WAV file and the one created after decompressing from FLAC, the EAC compare tool did not show any differences whatsoever. The foobar2000 Bit-compare tracks tool did not report any differences in decoded data, either. However, Duplicate Cleaner did not show the files to be identical.
No wonder, since when I checked their size, it turned out that the original file is 16 bytes longer than the one which emerged after compressing the original one to FLAC and decompressing it.
Taking all of the above into consideration, I believe the problematic file must have had some kind of glitch in its structure - the 16 additional bytes - beyond the actual audio data - which prevented Mp3tag from processing it correctly. FLAC-compressing and decompressing it resulted in the resolution of this problem - I hypothesize that it happened thanks to getting rid of the problematic extra chunk of the file.
If anyone encounters a similar problem with a WAV file, I would recommend trying the same (or similar) method - I think it should help.