You didnt mention your Roku model # or firmware version (Settings/System/About) or app version (highlight the app and press Options (*) on the remote).
Restarting clears the cache: Settings/System/System restart/Restart
But if you are going to do that, you should reinstall Discovery+ (if you havent already):
1) Remove the Discovery+ app (Highlight the app, press the Options (*) button on the remote, then Remove)
2) RESTART the Roku (Settings/System/System restart/Restart) <-- MUST DO THIS HERE!!!
3) Re-install the Discovery+ app
(If you dont restart after removal, settings/cached data associated with the app are left behind and it wont be a clean re-install)