Can't drag VST onto track

I’ve always started Cubase the same way.
Create Empty and add from there.
Today I tried dragging the thumbnail of whatever VST I was trying to use,
and it wouldn’t move. I tried another, with the same result.
I closed Cubase and reopened it. It worked, but did it again, and again.
Did I inadvertently change a setting or is this odd be behaviour?


Could you attach a screenshot, please?