It is not ticked.
It does however seem like it is working again.
1. I deleted my homepage.
2. I made a test-page, got the same problem, saved and closed the page nevertheless, opened it again, and voila – I could use the page builder without error messages. I got “Welcome to your visual preview area…
You don’t have any content at the moment.” instead of “Loading, please wait…”
3. I have made a new homepage and it now seems OK.
Have no idea why.