FIT file corrupted after transfer


For the second time in 4 days, my Edge 820 has failed me and output a corrupted file when transferring. However, I could browse the ride summary when I finished (but not afterwards).

The first time, the .fit file was clearly unrecoverable (only null bytes inside). But today, there appears to be a lot of data in it. However, all the usual recovery tools fail (reporting an invalid file). But I think it's only the header of the file that is wrong.

Do you know anything I could try to fix this? 

TIA in advance - best regards, Daniel