"Missing or invalid site address" error

Everything *was* working fine — but now all of a sudden I get a “Missing or invalid site address” error when I try to add a site (I’ve already created many dozens of sites with no problem).

I’ve changed a couple things recently — about a week or 2 ago I updated mapping via the backend (without realizing that this was a no-no)… and then I just updated sunrise.php manually + I created “mu-plugins” and moved domain_mapping.php into it (I had not done this originally).

It seemed to work OK, but I guess I must have screwed something up after all (?)

I am really stuck, confused, whatever…

:/