Hello there @Jan63, welcome aboard to the Fitbit Community! Despite the new Galaxy S9 is not listed among the supported devices in https://www.fitbit.com/devices it should let you to access the main Dashboard.
Usually this error is related to the internet access. If the error keep showing, try to swap your connection either from mobile data to Wifi or vice-versa. I have this error on my S8+ the last week and turning Off and On my Wifi was the way I was able to log in to my Fitbit app. It wouldn't hurt to give it a try to this workaround too.
I'll be around so don't forget to keep me posted if the issue persists.
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