"The App Service is not responding, try again later" for two days now.

I got my Flex box set up recently. It worked just fine for about one (1) day, and now it seems absolutely broken. Every time I click on the Apps button in the menu, I get the same error:

"We're Sorry. The App Service is not responding. Please try again later"

I've tried everything from unplugging and re-plugging, switching between using wi-fi connection and ethernet cable, restarting my modem, doing the system restarts in the menu- everything that I'm aware I should do to troubleshoot. Nothing has worked. At one point, I'm sure it said "You must be using Xfinity Internet for this box to work" or something to that effect, which makes no sense whatsoever.

Can somebody please explain what's going on? Because this is kinda ridiculous.

Thanks.