This is due to an error in the installation. You need to fix the admin_user_create() function in install/install.php.

https://sourceforge.net/forum/forum.php?thread_id=3045866&forum_id=779083

A MySQL connection is required before using mysql_real_escape_string()

You must move mysql_real_escape_string() after the connection to the database is made. It is entering in an empty string for the username and password because mysql_real_escape_string() is returning false or nothing.

function admin_user_create($username, $password) {
global $_ERROR, $db_inc_file, $table_inc_file, $mysql_db_inc;

include($db_inc_file);
include($table_inc_file);

// connect to the database
if(!database_connect($DATABASE_HOST, $DATABASE_DB, $DATABASE_USER, $DATABASE_PASS)) {
return fatal_error();
}

// clean up input
$username = mysql_real_escape_string($username);
$password = mysql_real_escape_string($password);