In the past few weeks I've got occasional reports of failures while uploading to garmin connect. Some were due to maintenance activity on their website, but in a few cases it was different. Looking at the transfer log, it seemed like the server was responding in a different way than usual. The problem tended to "fix" itself, meaning the guys at GC are probably A/B testing some changes, and you get a failure only if you end up being randomly chosen for the test.
Anyway, looking at the logs I've devised a potential fix. I have to say potential, because right now I cannot replicate the original failure anymore (that's the problem when the server is doing random testing). It will be part of the next release (Sportablet 6.9 / Uploader for Garmin 6.1).
If, after I release that version, you find that you still have problems, please get in touch using the contact page, I'll need to see your transfer logs.
Meanwhile: if you want updates on this issue, please follow the sportablet account on twitter or facebook.