I have this problem all the time using GC 3102 with my Garmin 510 to import .fit files.
My workaround is to...
1. With the Activity selected
2. Using the Edit screen add a 'Cadence' column
3. Copy the contents of the Run Cadence column to the Cadence Column
4. Delete the Run Cadence column
5. Save the file
6. Quit GC
7. Start GC