I have a multi-site with several child websites attached. All of them work fine except for one. I try to go to the wp-admin area, login and it says "You must log in to access the admin area." with no further data.
I tried disabling all the network plugins, but the page looped back into the login screen (I still cannot get into the backend).
All my sites use the same theme, but I tried switching to Twenty thirteen to make sure with no change. I cannot get in to disable the plugins for that page.