"* Are all files mp3 files?"
Yes, all the files are mp3 files
"* Are any files being bit-rate converted?"
Do you mean: after the sync to iTunes? In iTunes I can't notice any change of the bitrate.
Some of the files are VBR - if this could influence the sync.
After closing MM and deleting the related files in iTunes the sync works partially, i.e.
- sync of the tracks to a iTunes standard playlist: "Top 25" (that works for me because the playlist for the test has only 15 tracks)
- above mentioned error message appears in MM after the sync
- using a playlist with 90 tracks the files are spread in different iTunes-playlist - also to the "recently added" playlist
I am helpless