problem with new user registration

[carito]

Hello, during the registration process for a new user, once all of the fields are complete and the user click on "submit" to create the new account, I am getting the following error when the system tries to go to the "authentication.php" :

"The connection was reset

The connection to the server was reset while the page was loading.

The site could be temporarily unavailable or too busy. Try again in a few moments.

If you are unable to load any pages, check your computer's network connection.

If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the Web."

So, at the end the user cannot register. I am hosting my store in a shared server, with:

Debian OS
MySQL:5.0.91
PHP: 5.2.12

I am able to access all of the other parts of store without any problems. Based on other posts, I have a feeling that this could be associated with file permissions or worst case scenario the PHP version.

I have no idea what to do.

Any help will be greatly appreciated.