Paramount+ app won't stream on my cable box

I signed up for Paramount+ this morning. When I launch the Paramount app on my xfinity box, the app opens up, but very slowly. When I choose a program to watch, the screen just goes black. I went to the cable box Settings then to Privacy and cleared the cache, then powered the box down for ten minutes (unplugged it from the wall and co ax cable.) Powered the box back up, re-logged into the Paramount app, and when I chose a show the screen just goes black again. Now what? Please help?