Hi, exactly the same here also with a Samsung 6 (2017) series tv, it was working perfectly up to the update on the 8th of May since then all I get is the spinning app loading screen and nothing else. If you look at the app details it was released by "Now TV" so Samsung must not be responsible for this and as there is no way to roll back the version number I will just have to wait till its fixed.
Whilst you are awaiting for some further assistance from the NowTV Tech Team to sort this out, if you have a BT Smart Router I would double check if the smart setup in the BT Router settings is disabled and switched off.
This may be a pointless exercise, but whilst waiting for NowTV there is no harm in trying it.
If you are not using a BT Router or you have already disabled the smart setup feature on the Router already then please ignore this post.
More info here how to disable the smart setup on BT Routers.
I'm getting exactly the same with my Samsung UE40MU6400 since the app update the other day, it either spins the App Loading symbol for a couple of minutes or it just opens to a black screen. Tried everything I can think of - rebooting the TV, router, reinstalling the app, and removing the Samsung from my registered devices to free a slot up for it re-register but no luck. My Now TV boxes still work fine on my other TV's so it's definitely the recent app update that's borked it :'(