About 2 weeks ago, Fitbit changed how their data could be exported and Garmin quickly switched off the import of .csv and .xls files that came from Fitbit. Fitbit have now restored the option to export their data in the same format as before (as well as providing a much rich set of data as an alternative). Could you turn back on the Fitbit import feature you had before? It would be really useful and should be a very quick change.
Longer term, it would be great if you also import the richer data (minute by minute heart rates / calories / distance) data that Fitbit now provide. I understand that it more of a task!
Many thanks