Subgenre and comments keep changing

I'm having a problem. I'm not sure what software is causing this but hopefully somebody had a similar problem and knows how to fix it.

Two fields keep getting screwed up, SubGenre and Comment.

SubGenre keeps getting the field changed to one of the following: UBGENRE, ubgenre, UBGE, ubge, ubg. This usually happens to mp3's with a SubGenre specified but I've also seen it happen to files that never have had a SubGenre. This usually affects 20 or 30 files out of 13,000. It's not always the same files. I'll fix the ones that have been screwed up and then a few weeks later another group will have been corrupted.

The Comment field will occasionally (and randomly) be repopulated with a Comment that was deleted long ago.

Any idea what software is causing this? In addition to Mp3tag (always updated) I'm using WMP 11/12 (with MGTEK dopisp) and Fubar 2000. All of my tagging is done in Mp3tag.

I've read somewhere that WMP does not know how to cope with user-defined fields. Subgenre looks to me as though it is a user-defined field for V2.3
WMP cannot cope with V2.4 tags.

But as WMP tries to modify the rating field, it may cause problems.

As to your mysteriously modified comments: perhaps this is an aftereffect of WMP corrupting the user fields or you have some weird settings for reading and writing tags, e.g. that you read APE tags but do not write or delete them. There it could be that you read the APE comment, write the V2.3 comment which then is overruled by the (unmodified) APE comment (I hope this is correct).

That could be. WMP uses contentgroup instead of subgenre but I don't change star ratings in WMP. I change the ratings in Mp3tag.

My Mpeg Tags options are set to:

Read: ID3v1 and ID3v2 and not to read APE
Write: ID3v2, ID3v2.3 UTF-16
Remove: ID3v1, ID3v2, APE

My Ape, MPC... Tags options are all checked except writing ID3v1

Do these look like there would be a problem?

The settings look ok to me.
WMP and the ratings: unless you have saved the ratings in the field "Rating wmp" but have used the good ole "popularimeter" then WMP translates these settings into the field Rating WMP once WMP plays the files. e.g. a x|128 become 3. In this way WMP writes the tags anew which then might cause the problem with the user-defined fields.