whitescreen after login

hi, i installed today this theme (including the whole drupal cms): http://themeforest.net/item/evolve-multipurpose-wordpress-theme/7541690 i can sever over the site, but when i try to login a get a whitescreen. when i try to register i also get a white screen. the new user is to find in the database. the php log files are empty, the mysql log files are empty. what can i do ??? thank you for any helpl