I should clarify: I get that error when trying to access the site or the login page.
Thanks for the help.
Turns out I had neglected to transfer wp-activate.php from the root. Thought it wouldn’t be needed, as it was already an existing setup.
And, as if anybody was listening, I’ll finish by ranting about what a mess multi-site is. Really wish I hadn’t ‘upgraded’ to it for this customer. Plugins are all over the map as to if they need to be network activated or per-site activated. Upgrades don’t work. Yada, yada, yada… Somday I’ll get up the gumption to ‘downgrade’ them back to two domains or a subdomain. Less headaches, IMHO.