How are you combining? In GM? Try opening a single file in GPSmapedit and ADDing. Or try opening multiple instances of GPSmapedit and copying and pasting.
I am doing all the combining in GPSMapEdit, as I find it much more user friendly for editing trail types and using custom TYP files.
I combine in GPSMapEdit initially by opening one of the *.mp files and dragging and dropping each additional *.mp file until all map data is displayed, and I save the new group as a new *.mp file. Then I check overlaps, make slight adjustments, etc. I have also tried doing the same by using the ADD menu function, as well as opening each separate *.mp file, copying the tracks from each and pasting them into a completely new, made from scratch *.mp (in GPSMapEdit), but that file also always returns the level 0 error when attempting to export to IMG.
The only common thread I see is the *.mp files exported by GM will not successfully export to IMG from GPSMapEdit, while any *.mp file that is NEVER touched by GM always exports OK.
Unfortunately, GM is the only software I have that can extract the trails out of the OSM transportation data files that contain all road/trail/track/etc types in a single *.shp file, and GPSMapEdit is the only tool I know that can convert *mp to IMG format.