I had the same issue as everyone else with receiving the Xfinity / Roku -500 Error. We had been down for 3 days. I tried every tip.
The only one that worked for us was to reboot the router and modem. Once we did that everything came back. It made us log in to our Xfinity which to me indicated that it accepted our reboot. Our assumption of why this issue arises is to force us to reset our home systems every once in a while.
I'm not sure that you have to delete the Xfinity app to do the reset, next it happens I'm just going to reset the router and modem and see if it resets everything. Good Luck and hope this helps.