That error message is coming directly from your U-Verse box, not the Slingbox. I would try unplugging the U-Verse box and letting it boot back up; if that doesn't fix the issue, you'll have to reach out to AT&T and have them troubleshoot the cablebox itself.
Sling Media Technical Support
I have a Slingbox Model 350. Haven't had any problem until yesterday. Now every time I try to watch on my iPhone or my Mac, I get the above message after connecting. I have all the latest updates and plugin. Couldn't find any solution on the knowledge Base. Does anyone know of a solution to this problem?