I am having the same issue with Britbox. No other channels (i.e., Netflix, Amazon Prime, Sling).
Britbox initially said it was due to our old Roku2 player . I purchased a Streaming Stick 4K today, and same issue. Britbox will not open - it returns to the main Roku screen.
This started on Feb 8 - Feb 9. My wife emailed Britbox on February 9 about the issue, with the reply of "old Roku2 player".
There was an update at Roku or Britbox that is causing this.
- Deleted channel; Restarted device; Re-added Channel. Does not work.
I tried the "Delete/Restart/Re-add" on the Roku2 and the Streaming Stick 4K.
Britbox said they still work on the firestick. If Roku cannot figure this out I'll be moving to the Firestick.