Mine has started working again. hope it lasts this time

Only extra suggestion, which usually doesn't matter if it was just working yesterday or such - is log out of both apps, and close them.
Power cycle phone even perhaps.
Use web account disconnect on MFP, and revoke on Fitbit.
Log out of accounts.
Log in and pretend initial setup.

If it doesn't work once, no need repeating and just backlogging an issue that is going on.