I recently updated the application to v2.99a. It seems that somehow this version has difficulty (for the first time, ever) with Windows Media Audio files on my machine. I have a sizeable portion of my library in WMA form (mainly ripped CDs), and suddenly, with this build, I get a "File cannot be opened for writing."
This is a TOTALLY NEW BEHAVIOR, only introduced with this latest build. I've been using MP3Tag for several years now... as it's so dramatically nicer than trying to edit metadata within Windows Media Player.
I'm actually going to remove this build and reinstall an older one, to restore this functionality. But, it's mildly annoying to have to do so. Breaking one of the more common audio file formats... is a fairly major bug. And while I'm currently focusing on MP3 for portable and FLAC for high-quality these days, I really don't want to have to re-rip all of my audio CDs to be able to work with your program!