For some initial troubleshooting I would try disconnecting and reconnecting your Zwift and Garmin Connect accounts. If that doesn’t help, please try uninstalling and reinstalling your Zwift app.
Additionally, to address your corrupted .fit file, you could try using a fit file modification or fixing program, like Fit File Tools to resolve the issue. Before modifying any of your fit files via any tool, it’s recommended that you make a backup of the file or use a copy with any tool.
Here are instructions:
A. Find the affected fit file. You can either find the file on my.zwift.com or on your device.
B. Run the file through the Time Adjuster option.
C. Try uploading the file.
D. If the upload is unsuccessful, run the .fit file through the Corrupt Time Fixer. Avoid using the Normalize Time option on your first attempt.
E. Try uploading the file.
I hope this helps!