Have you tried going into your nowtv box secret menu by pressing the Home button 5 times, then quickly press the FF >> button 3 times, then quickly press the RW button 2 times on your NowTV remote.
Then once in the secret menu force a software update even if it shows you are on the same software version number.
If that doesn't work, maybe a full factory reset on the box, by going into the box settings menu and under Advanced System Settings there should be an option to factory reset the box.
Should you go down the factory reset route you will have to setup the box again from scratch (make sure you use you existing account details).
Same issue here on 2x Roku 2 (2015) and 1x Roku Express. Opening the app shows a "System update required" message when all three are already updated & restarted.
Oof, NowTV. Any chance of some complementary passes? We are all unable to watch a service that we pay for after all..