I have been trying for two days to upload a new bicycle ride from an Edge 810 to Garmin Connect, and when I do it says it is, "Uploading", but never uploads. I tried it on a Mac three times yesterday, and I tried it on a PC today twice, and always get the same result, it says it is "Uploading" indefinitely. I checked for software updates, and I am up to date. I am not using "Modern".
I don't own an Edge810 but I do own an Edge800 and I know that with my Edge800 that when my .fit files are written to the SD card and not the internal memory that they won't upload via GC - copy them from the SD card to the internal memory and no problems with getting them to upload.
Your other option, if your .fit files are indeed stored on the SD card, would be to upload them as a 'manual upload'.
Yes, this is a known issue with GC and has been for quite some time.
