Multisite Critical Error When Creating New Site

Yesterday I successfully used Multisite for the first time with no problems. Today I tried to do the same with a different WordPress installation but when I click the button to add a site I only get “There has been a critical error on this website.”

Then the site appears to only have been added partially. If I assign my user account to the new site manually then it shows up under the menu but with no title. Checking the database reveals nothing added that would suggest a second site. I have nothing to go by because the critical error message offers no tips.

I have tried wildcard and static subdomains. Same problem. On top of that, the second site links under the menu are not to it but to the first one.