As per the title, The Box is exiting Now TV when trying to play episodes of Flash or Arrow. Episodes of The Blacklist, Supergirl and Blindspot work fine.
I have rebooted the box (mains power cycle), and it makes no difference.
This looks like a similar issue to one that affected 24 a few weeks ago, where tweaks to the content files behind the scenes mean the box can't read them properly. Unfortunately the problem is with the content files at NOW TV's end, not with our boxes, so there's nothing you can do to your box to get it working again.
When the NOW TV Team pick up the post they will pass the info on to the content team to sort, but from past experience it will probably take a day or two.
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