When I’m trying to add my Polar Flow account on the Synced Devices page, I repeatedly get the message
“Unexpectedly unable to link your account.
Maybe Polar is having a bad day?”
If I deliberately use a false password, I correctly get
“Unable to login with the Polar email and password you set. Have you recently changed your email or password on Polar’s website?”
so a wrong username/password does not seem to be cause.
Tested in Firefox 52 on Suse Linux.