This problem occurs at my computer for about half a year. I don't remember what version I used then or if I have changed anything at that time. Maybe it was at the same time when i installed foobar2000, but I'm not sure.
I'm using now the latest mp3tag version (v2.46a) on Windows Vista but the problem also occured with the versions before.
The problem occurs only sometimes when I change files and save them. I happens both when a source script is saving and when I save manually. When I save more files at once, the problem occurs typically only with one or two of them.
I have the impression that it occurs more often with larger files (50MB+) but this could be only my feeling.
I also have the impression that there are times when the problem doesn't occur for hours and times when it happens at every tenth file. But this could just be a coincidence.
The problem occurs while the "Writing tag data" message field is shown in mp3tag. When it occurs an error message pops up which says:
File "C:\path\filename.mp3" cannot be opened for writing.
Do you want to continue?
When I click yes, saving goes on with the other files. When I click no, the other files remain untouched.
In both cases, the "error-file" is tranformed into a .tmp file like "TMPxxxx.tmp" (x can be a number or a upper case letter).
I guess when the problem occurs, the saving process doesn't finish properly. When I change the ending of this file back to .mp3 manually I get the working mp3 file with the tags changed the way I wanted them.
When I watch the files in a windows folder while they are saved with mp3tag, the file names all change shortly into a tmp file and then back to mp3. My "error file" get proper changed ID-tags and the audio information isn't damaged in any way. The just don't get their name changed back to mp3.
There is also a second less often scenario of this bug where the file isn't transformed but keeps unchanged with old ID-tags. Instead there is an additional .tmp file which turns out (after being changed to mp3 manually) to be the mp3 file with the tags as they should be after saving.
As I said I have no idea why this happens sometimes and sometimes not. I don't think it happens with certain problematic files. When I got the unchanged files as written in the second scenario, I often run the source scripts a second time and it works without error. I also got situations where I had the same error again with the same file of a album for two or three times (creating three identical tmp file). But after another run it suddenly works.
The only assumption I have is that the problem could be caused by foobar2000. Foobar is constantly monitoring my mp3 collection and updates its library within seconds after changings. Maybe this monitoring involves a process which accesses the file in some kind which keeps mp3tag from saving them because they are opened by another program.