If still happening, try the beta version...
Uninstall the present version, then visit the app page in the Play Store, scroll to the bottom & opt into the beta program. Next, reinstall the app (which will install the beta this time) & sign in to check if it's ok.
There appears to be a bug in the NowTV device registration process with Android Nougat 7.1.1 devices. If you experience 'you have been signed out' error when trying to play content, see if your device is actually appearing on your account (my devices). If not, there's the issue! I reported it today to the chat support people, and they said they'd report it to Tech support
The NowTV support desk have replied saying they have escalated my case ref 06632217, so will update accordingly