I have recently registered my mobile phone as my 2nd device, when trying to watch sky sports & sky cinema on their respective apps on my phone, the warning comes up that I have reached the maximum registered devices - everything is ok on my other registered device, which is my tablet.
Whenever there is a change to your OS version on any device, this can sometimes trick the TV Anywhere into thinking it's a brand new device, when in all fact it isn't. Has there been any change to any of your devices lately?
Re-registering your device would resolve this and if you can take a look at your allowances, I have refreshed them for you
Please let me know if this helps and all the best. Ben