Me too. The (.fit) file in question contains about 14 hours of data - I can't upload it to GC properly (eventually managed to do so via manual upload, but the map, various views (heart rate, elevation etc. and the player do not load). I now can't delete the activities (more than one through several attempts to upload it) from GC either :(
The "Large Garmin 500 Fit file gets internal server error" posting in the Troubleshooting part of the forum seems to describe the same problem.
Can anyone help?