Maybe something underlying was changed? Bluetooth perhaps?
My Polar M430 running watch has been syncing perfectly for about 3 years and suddenly the "weekly time spent in heart rate zones" doesn't carry over to Polar Flow (their data viewing web service). I had the problem for 2 weeks. Seems OK now.
Nothing mentioned on the Polar web site about anything. Which I think is being very stupid bunnies, because that way support will get about 10 million emails about stuff they already know about and maybe they should be fixing the issue rather than responding to mails.