Have you tried to delete and reinstall the app?
This link will give you general information for troubleshooting on the box.
I have had exactly the same problem with mt 3 active NowTV sticks. Like you I have tried all the fixes recommended, and with a similar negative result. This is clearly a problem with the latest NowTV firmware, as that's when the problem started.
Hopefully someone at NowTV or Roku will read this and do something about it.