I am experiencing this same problem. I have a few things to add.
I have 3 Roku's and this is the only one with the problem. Sling has never been able to update since I bought this unit. I just always figured it would eventually "fix itself" via a Roku update or something. As you might guess this one is not used very often. So, using the old sling interface has not really been an issue until today. The sling app is stuck at 7.15 build 367. Roku model 4801RW - Roku Ultra is at 10.5.1 build 4066 DE.
I guess that Sling has put out an update that makes the old interface obsolete.