Blank page at admincp

[TheAdminMarket]

Hello,

I'm getting a blank page when I'm trying to access directly the admincp (..../admincp/). I'm saying directly because I found a strange way to access it, like:

1. Login from frontend
2. The system recognizes me as Admin that's why the links "Mod" and "Admin" are active.
3. Open in a new tab the Mod link. System asks me to login. So now I'm logon to Moderator CP.
4. From the frontend page again I'm opening in a new tab the "Admin" link. Now without any other login form I can access the Admin CP

This is the only way that I can access the admicp. In all other cases I'm getting a blank page.

PHP 5.3.3
vB 4.2.2 Patch Level 1

Thank you