After installing 0.18.0 as per the install docs, I tried to log in as the default user (administrator/root). This is what I get:

Warning: Cannot modify header information - headers already sent by (output started at /home/httpd/vhosts/<domain>.com/www/bugs/core/html_api.php:139) in /home/httpd/vhosts/<domain>.com/www/bugs/core/gpc_api.php on line 220

Warning: Cannot modify header information - headers already sent by (output started at /home/httpd/vhosts/<domain>.com/www/bugs/core/html_api.php:139) in /home/httpd/vhosts/<domain>.com/www/bugs/core/print_api.php on line 37

Warning: Cannot modify header information - headers already sent by (output started at /home/httpd/vhosts/<domain>.com/www/bugs/core/html_api.php:139) in /home/httpd/vhosts/<domain>.com/www/bugs/core/print_api.php on line 39

Warning: Cannot modify header information - headers already sent by (output started at /home/httpd/vhosts/<domain>.com/www/bugs/core/html_api.php:139) in /home/httpd/vhosts/<domain>.com/www/bugs/core/print_api.php on line 40

Warning: Cannot modify header information - headers already sent by (output started at /home/httpd/vhosts/<domain>.com/www/bugs/core/html_api.php:139) in /home/httpd/vhosts/<domain>.com/www/bugs/core/print_api.php on line 41

Warning: Cannot modify header information - headers already sent by (output started at /home/httpd/vhosts/<domain>.com/www/bugs/core/html_api.php:139) in /home/httpd/vhosts/<domain>.com/www/bugs/core/print_api.php on line 42

Warning: Cannot modify header information - headers already sent by (output started at /home/httpd/vhosts/<domain>.com/www/bugs/core/html_api.php:139) in /home/httpd/vhosts/<domain>.com/www/bugs/core/print_api.php on line 46

The same thing happens if I try to make a new user and log in with it.

After some research, I discovered this is probably because of a problem with output buffering (the output is started before header() is called, so you need to buffer the output). I checked my phpinfo() and noticed output buffering was off. So, I turned it on, and restarted apache. Now, when I try to log in (with either my new user or as administrator) it just goes back to the login page. If I enter a wrong password or user it also returns to the login page but gives me an error. (So, I go to http://www.<domain>.com/bugs/login_page.php, enter in the correct username/password, press 'Login', and it takes me to http://www.submersion.com/bugs/login_page.php?return=%2Fbugs%2Flogin.php, which is the same login page).

These are my particulars:

Gentoo Linux on a dual Opteron 240.
mod_php-4.3.4
mysql-4.0.16
apache-2.0.48

Any ideas?