This is my case.I created a multi-site installation where users can register and be provided with a subdomain. As of this time, the administrator has not yet enable the creation of sub-domains in the apache configuration, so I manually added the newly created subdomains (from my testing) to my hosts file.
After registration, it works but it will only access the front end. If I will logged-in, e.g. http://mysubdomain.mymultisiteexample.com/wp-admin, it won't allow me to logged-in to the backend even though the logged-in details are correct? Any clues what is causing this? Is this related to a cookie? Thanks for any ideas..