Saving Terms of Service gives me "Page not found"

When I try saving the Terms of Service I'm getting a "Page not found" error at
mydomain.com/wp-admin/network/settings.php?page=signup-tos

Just for clarity, I can to go the Terms of Service option page and I can see what I've previously entered in the TOS text box but when I try saving my changes, I get the error as above, and the URL is the one I pasted above.

The TOS options page worked initially (yesterday) and saved what I entered, so I'm not sure what's up now. I'm trying to remember what I changed since then. I have set up Multi-DB at some point in there, though I don't remember if it was before or after working with TOS, and I have installed Pro Sites.