It is looking at the moment as if the problem is related to W3 Total Cache. At least I have reactivated all the other plugins except this, and the font selector is visible. When I activate W3 Total Cache, the selector disappears, and I can toggle the effect by deactivating W3 Total Cache again.
When I get time I’ll see if I can identify a particular setting in W3 Total Cache which problematic.
Yes, I’m not totally convinced W3 cache is the real culprit. I wonder if there is some kind of memory limit somewhere which is being exceeded and whether it is not a specific plugin at all. At another point, I felt sure it was a different plugin, but couldn’t reproduce it.