I've upgraded from 2.2.4 to 2.3.0.
In 2.2.4, adding a block was a matter of picking from a list. For example, the drop-down menu gave 13 different types of blocks I could add under the "System" headding.

I want to add some similar blocks under 2.3.0 but all I can see is a choice of the position.

If I want to add a block like "themes", "user menu", or "Who's Online" in 2.2.4, how do I do that in 2.3.0?

The problem is gone following an upgrade to 2.3.1.

Original cause not determined.