Line 109 of wp-admin/admin.php
Wordpress try to set the memory_limit to WP_MAX_MEMORY_LIMIT
This request coul be blocked Hosters, or by use of suhoshin, so Please considere that the memory limit of 256M by default is verry too up.
The blocking of this request make server not responding and turn php into a infinity loop. (limited by max_time of course)