Subdomain based multisite login gone

I have a subdomain based multisite with a couple of subsites. I just upgraded to WP v 4.0 and now cannot access the login pages of the subsites. Browser cookies and cache clearing did not help. Main site is accessible no problem and subsites are on the dashboard menu. When trying to access the dashboard on the subs, I get a Not Found error on one and a continuous list of posts on the other. No admin access in either case. When I create a new subsite, I cannot access the admin page either. All was working fine prior to the upgrade.

Help?