I find it tends to come and go.
Sometimes, I'll see it maybe three times in a week, other times, not at all for several months.
It appears to be a false positive.
Because I never see it when surfing the net outside of TOL.
Also, because what I do now is ignore it, and simply go back a page, and then go forward again, and that often works - as if the thing had merely been a momentary misread on the other end.
By the way, if you'll Google image the words <certificate error> you'll see all sorts of versions of this, together with all sorts of solutions.
Could also be related to ads placed in the page. If it's intermittent, that seems more likely.