do you just see your phone and box listed? If so you shouldn't get any error messages when watching on those devices.
If other devices are being added to your list and there are others in the household it may be that someone else is watching on those devices as well. Very rarely if your phone or box updates software it may be recognised as a new device as well. If anybody else watches on their phone it's possible to delete devices direct from the NOW TV app so check nobody is doing this without realising.
To get it sorted quickly so you can watch again, best to pop onto live chat and explain what's happened. To reach live chat go to the following help page