Hmm, interesting. Starting to look like something wrong with the files. I'd be curious to know what the files look like, both before and after the "fix" in MP3trim. I think that if you can do this consistently, that the empeg guys might like to see one of the troublesome files to see if there's something wrong in the importing engine. They are working on the future Rio product lines, and I'm sure they want their MP3 importing engine to be as robust as possible.

Do you ever get these errors?

Nope.

Any suggestions on what to try next?

Start investigating what might be wrong with the files themselves. What's your encoding and tagging procedure?