Awesome troubleshooting
Feel free to clear the Fitbit App cache on the phone, all your data stays intact, the fitbit app will recreate the cache when syncing with your account.
Sometimes installing new app updates over existing ones, while using the cache for the previous app version(s).. can cause bugs / glitches.. like a app not loading, or crashing..
Check out the Android Central Article (Credit to author's of the website / article):