You didnt mention your Roku model # and firmware version: Settings/System/About
Try reinstalling the app:
1) Remove the Warhammer TV 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 Warhammer TV 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)
Otherwise, you'll need to contact Warhammer+ about issues with the app/service: