it's because you use coniserver.net domain while you are visiting forums, and then when you click something, forum goes to it's new url(opserver.de) which your cookies are not valid on this domain.

So if you login while you are on a opserver.de page, it'll create cookies for this domain.

Now you have cookies for both of the domains, no matter which one you visit you'll see yourself logged in.

Set your bookmarks to opserver.de since it looks like the new adress we'll be using offically.