Please help! Corrupt .fit file

In desperate need of some Garmin-expert help!
This weekend I took part in a 315km bike race, I had an activity started and saved it after I finished the race, all good so far. But the acticity wont synk to the Garmin Connect app! I´ve tried uploading it manually but keep getting an error message that the file type is not supportet. The Garmin support team says it probably is because the file is "corrupted" and that they can´t help me but that there might be some programs that could sort it out?!
All the data such as pulse, calories, active minutes etc has synked to the app from the watch and I can see the activity in the watch - but it wont synk to the app.
This is my longest ride to date, I´ve never had this problem before and am honestly quite upset over the fact that I cant synk the activity. Can anyone help?