If you get a 500 error code, the contact links probably are not properly set in Joomla! when SEF is set to 'ON' and the menu item is missing.

For the contacts to show properly, you need to have a menu item where your contacts would show. Create a menu item that will show the category (or all the categories if your contacts are from several categories). You probably don't want this menu item in your menu but it needs to exist in order for the contacts to load in a page fit for them. Add .item-xxx {display:none} to your template stylesheet. This way, the menu item won't show (xxx is the menu item ID for the menu item created).