Looks like the backend web app is down not allowing you to login. The App doesn't really do anything execpt put a wrapper on the xfinityhomesecurity.com website.
Apparently the entire home security web portal is down and thus the app is as well. I was told it'd be fixed as soon as possible (by morning) by a chat rep. Keep your fingers crossed, but don't hold your breath
The alarm functions are independant of the app, you'll be able to arm/dis-arm from your panel, keypad or remote. You just loose the ability to view the current state and arm/dis-arm from the web. Even if the internet fuction is down on the wireless base station it has cell backup which is used to phone in the events.