Sync Failed - Server Error

My f5 and/or GCM are being a total PITA today.

I charged my f5 today, and since then it's just refused to sync with Garmin Connect Mobile. I get a "failed to sync" red banner on the bottom of the app and there's a red circle with an exclamation mark next to it in Settings>Devices. If I try to sync it from there, I just get "Sync Failed - Server Error"

I've tried turning the f5 off and back on again, removing it from GCM and re-adding it (which took several attempts as it kept saying it couldn't connect to the Garmin servers), deleting GCM from my iPhone and re-installing it, even updating my f5 to the new beta, but still it refuses to sync.

The odd thing is GCM does seem to know how many steps I've done, so some of the sync must be taking place, but then it just fails and stuff like HR isn't showing in GCM.

I know the last resort is to do a hard reset, but now I've finally got to the stage where my VO2 maxes and Training Status data is beginning to come together, I REALLY don't want to have to go back to Day 1.

Any ideas?