I have the same problem on two different computers. The file system is ok.
I removed mp3tag and all its parts with Revo Uninstaller and reinstalled version 2.87a.
The problem stayed. I tried a smaller amount of files, in some cases it works, in some cases it did not. On a network drive all files could be saved. That's a total different file system.
The file with a problem all started with "(", the files where mp3tag worked normally started with a track number without braces. On the network braces are no problem.
Saw another problem, which was also related to a brace at the beginning. Using the "filename to tag" function does not work either (no example, Ok button grayed out). Removing the "(" solved the problem.
90% of my collection has filenames starting with a track number between braces. I don't know why this version has problems with a starting brace. I'm using mp3tag for a many years and never saw this problems.