Hi
I accidently deleted a training session I did today from Garmin Connect and I got this message when trying to import it from my garmin 735xt
If you did this - How Do I Manually Upload an Activity I Recorded to Garmin Connect?
then get that message, there is a problem with the .FIT file being corrupt. Attach it to this thread. We can take a look to see if it is repairable.