"500 - Internal Server Error" when I try to create a new account; lighttpd; php5; mysql5;

Hi!

This is my system:
• Ubuntu 7.04-server
• lighttpd 1.4.13-9ubuntu4.2
• PHP5-cgi 5.2.1-0ubuntu1.4
• PHP5-common 5.2.1-0ubuntu1.4
• PHP5-mysql 5.2.1-0ubuntu1.4
• mysql-server 5.0.38-0ubuntu1

I've tried to install Drupal on my testing server. First, all works fine, it installed correctly and I can see the "Welcome to your new Drupal website!" page.

However, when I try to create a new (the first) account and I click "Create new account" after I entered my user data (Username and E-mail address) my lighttpd says: "500 - Internal Server Error". The error message also appears when I try to request a new password.

This is what the lighttpd error log is reporting:

2007-09-16 17:57:22: (mod_fastcgi.c.2431) unexpected end-of-file (perhaps the fastcgi process died): pid: 7013 socket: unix:/tmp/php.socket-1 
2007-09-16 17:57:22: (mod_fastcgi.c.3214) response not received, request sent: 1200 on socket: unix:/tmp/php.socket-1 for /drupal/index.php , closing connection

The www-data user has full permissions (7) on drupal.

Unfortunately, I have no idea what I can do. Probably I've forgotten to install/activate a module or something?

Thank you for your help. :)

Dittboy

p.s.: I hope you understand my bad english. :/