"Add content" (/node/add) gives an error

I'm in the process of migrating a Drupal 7 site to Backdrop.

When I click on a navigation link to /node/add "Add content", I get the message:

You have not created any content types yet. Go to the content type creation page to add a new content type.

Excuse me? Not only are there the default node types "Page", "Article" etc. but there's also a custom node type made in Drupal 7.

Clearly something is broken. I wonder how to fix it?

 p.s. If I go directly to a creating page like /node/add/page, I can enter a new page, so it's not a permissions issue, as far as I can see.