This problem occurs in a modified OSCommerce shop.
The problem has 2 parts.
1) If you enter your email and use the mouse to click on subscribe get this error (see picture1).
Warning: mysql_close(): 206 is not a valid MySQL-Link resource in /var/www/vhosts/[url removed, login to view] on line 179
2) If you enter your email and press enter to subscribe you get a different error (see picture2).
The requested URL/[url removed, login to view] was not found on this server.
If we check in the database, you'll see that there isn't a new subscriber added.