wp-login not functioning on main domain

Hi
I’m working on a multisite deployment at https://equnus.com with one subdomain blog at master.equnus.com. I’m using master.equnus.com as a ‘master blog’ for replication with plugin ‘blog copier’.
The https://equnus.com/wp-login.php (I’m using appthemes Classipress so admin login is from the front end) suddenly goes to a randomly generated page showing an advert from the front page, tiled and nothing else, and I’m unable to login via this URL or wp-admin. https://master.equnus.com/wp-login.php works fine enabling access.
In a normal WordPress deployment I would have immediately assumed plugin conflict but Multisite works from one WordPress instance and one database? even so, I have de-activated all plugins, this has not worked and I wonder if anyone has any ideas as to the cause of this.
Many Thanks
Nick Garnett