I want a custom login page
 

Hi,
is it possible to have a landing page which only contains the login form field for registered users and a button "register/sign up" for all those who want to register?
and only after the user logged in correctly he will be forwarded to the forum respectively will be forwarded to the registration process.

You can control the visibility of each module per usergroup in the Module Configuration modal in Sitebuilder. You can add a Static HTML module with the same Login iframe used in the original login dropdown. Then make it only visible to Unregistered Users usergroup. Then the rest of the modules on the page make them visible to all usergroups except Unregistered Users and other usergroups as desired.

Thank you, thats almost it.
But it stills shows the navigation and sub navigation header, altough you can not click on the menus i want them also to be invisible.

Were you referring to the navigation bar on top below the site logo? Nav menu items visibility can also be controlled per usergroup in Sitebuilder > Navigation Bar