I am also having the same issue. I have sent a request to Garmin Support. Will wait to see what they say. I know that maintenance was performed on the site, just wondering if this is some fallout from that.
I have had the same issue, (using BC1000 and 910xt) the workaround is to have an Activity file to upload as well, if you upload the Activity file, then manually upload the Health data it should upload ok. If you have no Activity starting and stopping the unit after a few seconds, uplod the files and then deleting the file in GC works...stupidly frustrating!:confused: