The last two times I've started the computer, I get a pop-up message which reads, "Critical Error. Start menu and Cortina aren't working. We'll try to fix it the next time you sign in." Then there is a button reading, "Sign Out Now" to click. I don't click the box but reboot the computer and it starts up fine.

The box is green with no icons.

I downloaded Windows 10 about 2 weeks ago, fyi.

Does this seem to be a legitimate warning, or what's going on?