I put this in How it Works but didn't get any answers. So I'm trying here instead.
When I turned on this morning, I got an orange message in the bottom right hand corner saying something about having to validate the mse certificate or in would terminate in 30 days.
I pressed the validate now button and now I have a little box on the left saying 'legit(2). hta.
It's probably a time issue, and seeing as a leap second was added last night which caused all sorts of problems all over the internet I'd guess it's something to do with that (although check your computers date and time just to make sure)
as long as is validated OK and MSE is green and saying you're protected you can almost certainly ignore it (unless it comes up again tomorrow)