Similarly on a Pixel 4. App updated 9 hours ago during the night. I'm on the beta program (3.25 beta). App crashed repeatedly on startup. Cleared cached storage, same result. Cleared all storage, and once credentials are entered a "Please wait..." message is displayed with the spinning circle animation. This message has been displayed for ~5 minutes now with no change. Hope this helps.
Samsung note 9 with android 10. Same here too. Unista/Reinst. No diff. Fitbit app wont let me login. Called help. Didn't help. On line chat is down now so I think they know they are down.
Same for me. I deleted the app and reinstalled, and now I can't even log in. It isn't crashing now but it isn't responding when I try to log in. I checked @FitbitSupport on Twitter and there's no notice I can see.
Samsung S10+