upload fails

P.S.
you need to delete the activities not just from your 310xt, but also from your computer! If you can't find them, (like me at first) it is probably because the files are hidden.
I have to go make dinner right now or I'd try to explain further! but I found the how to's here on the forum & also with Google.
will try to check back later...good luck! you can fix it with a bit more persistance :)

OK, back again...
what we 1st did, and then were able to manually upload to GC was follow this path-->
Computer/C:/Users/OEM -* or whoever you are using*/AppData/Roaming/Garmin/Devices/*your devices serial#*/Activities
and deleted the activities (FIT files), then stepped back one ( to your devices serial #) and then/History
and deleted the TCX files
Then, when in GC , chose upload file, manual upload, followed the same path and uploaded the chosen new, uncorrupt TCX files ....
Then we also clicked down the same path one more time but veered to Garmin connect/pending uploads and deleted those and now our 310xt is successfully transferring files to the AntStick which then passes them on to Garmin Connnect like it used to :)
the path--> Computer/C:/Users/OEM -* or whoever you are using*/AppData/Roaming/Garmin/Devices/*your devices serial#*/Garmin Connect/Pending Uploads
I am writing this down here for my benefit too, in case I have to do it again but forget how!! what a major PITA it has been to figure out!
Hope this works for you too (it should)

Thanks all, It realy works. I moved all fit and tcx files from the ant device folder (activities, history) to an other place. Now I'm able to upload single files manually.

But that would mean, that the garmin communicator plugin reads all activities in the device folder, even when I want to upload a file from a different directory or device!
Why garmin?