Gutenberg Blocks, add block button greyed out/inactive for Pages and Posts

flips — Wed, 15 Dec 2021 19:59:28 +0000

I was going to give the Blocks editor another try.
But for Pages, and for Posts, it doesn't work, there's no blocks available, the plus button is greyed out:

If I click the tre vertical dots on the right and select Block Manager, there's lots of blocks, and they're all enabled ...
And the Blocks editor actually works for widgets stuff ... Any idea what to check or do?
(This is a Multisite WP installation, latest stable WP version.)

Gutenberg Blocks — LowEndSpirit

Gutenberg Blocks, add block button greyed out/inactive for Pages and Posts