There was an error in some of jbensman's transparent maps (such as MyTrails) that caused problems for some people before. I think he has fixed this now, but if you are using his maps then it might be related. IIRC, Mapsource worked correctly to install the map but Basecamp/MapInstall had issues.
If you are using Basecamp/MapInstall then try Mapsource and see if it makes a difference.