Safari won't open Facebook

Using Safari 11.0.3 (13604.5.6) on High Sierra 10.13.3 (17D102).

I try to open Facebook from a well-used shortcut and get this:

"Safari can't open the page "www.facebook.com" because Safari can't establish a secure connection to "www.facebook.com""

I can log on to Facebook perfectly well on my iPad using Safari, and also via the Facebook app.

Why can't I use Safari on High Sierra to access Facebook? It was working perfectly OK a couple of days ago...