Problem

When you log in, you are actually not logged in and login screen is shown again.

Solution

You need to explicitly tell PHP where to store session data because it is not configured properly. In most cases it'll be /tmp directory on your server, but in some rare cases you'll need to create writable (CHMOD 777) folder on your website and to change /tmp to the FULL path to that writable folder. You can use phpinfo(); to find out what is full path (DOCUMENT_ROOT) of your web site.

If your server is installed as server module, and not as CGI module you could try to create blank text file, name it .htaccess and place it in your website root folder, and place following lines in it. If such file already exists, just append following code at the end of it:

php_value session.save_path files
php_value session.save_path "/tmp"

Otherwise, if PHP is installed as CGI module create blank text file, name it php.ini and place it in your website root folder, and place following lines in it. If such file already exists, just append following code at the end of it:

[Session]
session.save_handler = files
session.save_path = "/tmp"