I encountered the white screen today while uniting WP 3.6 and phpBB 3.0.12, which only appears when user intergration is turned on. The problem is caused by WP and phpBB both having a validate_email() function, causing PHP to throw a redeclare error.

Soluton: validate_email() is actually deprecated on WP 3.6, so you can simply delete or comment out the ‘validate_email’ function in [wp-includes]->ms-deprecated.php.