I’m adding a comment to this issue that might help cases where Zwift session connection was lost during a ride and attempt to manually upload the fit file to Strava fails with “upload appears to be malformed…unable to process”.
This happened to me today (my fault as my laptop ran out of power), and I tried using the FIT File Tools/Corrupt Time Fixer option to fix the file ,but, it still wouldn’t upload. I tried that option 3 times with no success.
Then, I started & completed a short 2 minute Zwift ride, but, I didn’t upload it. Instead, I used the FIT File Tools/File Combiner option to combine the “bad” fit file (first ride that was interupted) with the “good” fit file (2 minute ride ), and it then uploaded successfully. The ride can be trimmed as needed once in Strava… Note; It connected a straight line from my last location when I got disconnected … back to the starting line of the 2 minute session. That’s the part that needed trimming.