To workaround the problem, the admin user needs to fix issues on the database level. On the Atlassian Community the post Confluence add Space fails - An unknown server error has occurred. provided a solution to a similar problem that also fixes the issue we have encountered.

In fact, dropping the AO_54C900_SPACE_BLUEPRINT_AO after server stop, removing the plugin cache directories, and restarting the server also fixes the problem: new spaces can again be created.

There is also a knowledge base article Creating new page or space loads forever that seems related. But the described problem in this article is that the process for displaying the space wizard dialog loads forever, whereas in our case the process terminates and shows an error message.