My Garmin 800 works perfectly fine with the old website too but does not connect with the new one. If you click on the profile icon on the left hand side of the new website it has an option to switch to the old version temporarily. Not sure what I'll do tho
Do you have Garmin Express installed? If so, you might have to remove the Edge 800 device from Garmin Express, and re-add it. According to some support posts, you may need to do a factory reset on the device as well.