I had very similar problem this morning during upgrade 1.2 -> 1.3.1
After some successful SQL setups my browser was redirected to admin/index.php but no output was produced by server
Now I've found that problem was in included file lib/blocklib.php. It is new in distribution and I had forgotten my Linux group set to 'users' with umask set to '0022'. So my web server (running as group wwwadm) couldn't access this library and several other files - with no error message even after putting error_reporting(15).
My solution: be sure that web server really CAN access all files in your moodle directory
