Ok, maybe it can be helpful to somebody else.
Seems the app sometime just "forget" data and can't login anymore.
To solve that, this is what I did and it worked, the app is now working again :
- go in the app management, select the app and erase the cache
- then uninstall the app (yes, the cache should be erased when uninstalling the app but in my case did not happen, I had to uninstall the app twice after finding the cache not correctly erased)
- restart the phone - i.e. force the phone in a stable known state
- after restarting the phone, reinstall the fitbit app
It should work at this point.
There was something in the cache that was forcing the app in a wrong state.
Hope it helps somebody else.
Now the app is back working as expected.
Best Answer