Update: This issue should be fixed as of the January 8th update.
When Zwift tries to automatically upload a ride greater than 2km that was successfully saved, it may show up on Zwift as a "partial ride" and fail to automatically upload to third-party sites. Manual uploads of these files to all third-party sites may also fail with a message similar to "the upload appears to be malformed, and we're unable to process it." The ride may also show up as two .fit files.
We're actively working on this issue and will update this post with more information.
While we're unable to upload .fit files to your Zwift dashboard or add missing miles to your Zwift account, there's a couple different workarounds that may help salvage part of the ride for tracking on any third-party sites. Note that it's likely you will lose some progress from the fit file as a result of this issue.
1. Use a fit file modification or fixing program, like Fit File Tools*, to correct the timestamps.
2. Download the file directly from one third-party site and upload it to the other. If it uploads to one site (like Garmin Connect), but not another (like Strava), you can download the file from one, and manually upload it to the other.
Example: if the file uploads successfully to Garmin Connect, but not Strava, you can download the file from Garmin Connect and upload it to Strava.
Example Instructions for Fixing a file via Fit File Tools*
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.
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.
*https://www.fitfiletools.com/ is not an official Zwift site, nor directly affiliated with Zwift. These instructions are only provided here as an example, and any use of the site is at your own risk.