I’m having a bizarre issue with using the block editor. When I try to add a block in Elements, this is what the screen looks like:

There’s no +icon anywhere!
Same if I try to use the block editor to create a page.
(Not sure if it’s related, but toggling “Top toolbar” in the view menu doesn’t seem to have any impact. Something is messed up in the header, perhaps?)
I am new to using the block editor, so maybe there’s something incredibly basic that I’m missing, but for the life of me, I cannot figure out what it is.
BTW, disabling all plugins (except for GP Premium and GenerateBlocks, of course) makes no difference. I’ve cleared the cache, upgraded to the latest version of WP (5.7.2; I was on 5.6.4 before, when I noticed the issue), and tried using a default theme. Nothing helps.
Any ideas what’s going on here?