Did you get the problem fixed?
I ran into the same issue. Here’s what worked for me:
I went into wp-config.php and changed SUBDOMAIN_INSTALL from “false” to “true” (without quotes, obviously).
For some reason, this let me login to my /wp-admin/ again. But then I could only add new sites as subdomains, and I want subdirectories. So I went back to my wp-config.php file, changed SUBDOMAIN_INSTALL back to “false” again, re-logged in, and magically — everything now works.
