OK, I imported by 58 tracks (GPX) into Mapsource. They all show up fine as track lines (at same time) in the Mapsource map window and list under the Tracks tab. I deleted tracks 3 to 58 and sent the remaining (first) 2 of 58 tracks across from Mapsource to Mobile PC (at same time) and both OK. Mobile PC sees them fine.
I repeated the process (deleting tracks 1 to 57 from within Mapsource) and then sent them across. Mobile PC bombed again with same error and won't open any more. Briefly flashes up "Importing tracks" and then stops for few seconds and then comes up with the error and won't reload.
I deleted only strk.nvm and current.gpx in the GPX folder and still gives the same error message and won't start.
I then deleted the entire GarminMobilePC sub-folder within Roaming>Garmin and Mobile PC starts up OK (but unlike a normal start, it goes through its initial set up screen as when run for first time including:
"Did this purchase of Garmin MObile PC include a Garmin GPS receiver?" (yes/no)
"Connect or power on your Garmin GPS receiver now" (back/done)
"To learn about features, updates, special offers, go to..."
Then the acquiring satellites screen (with option to skip)
Then the "Warning use GPS safely etc" screen (part of the normal intro)
Bandicoot