Ok, I have managed to get MSM to work.
Now I have my EE installation at A.com and a sattelite installation (only consisting of path.php and index.php) at B.net.
I have setup a forum and chose Site B as Site for forum. Furthermore, I have set the forum URL to: http://b.net/index.php/forums/
If I know open that URL it gives me an error saying “Unable to locate the forum theme folder.”
Why is that? Do I have to create a theme folder within the documentroot of Site B where index.php and path.php are? Or maybe, is it necessary to create an open_basedir rule allowing site B’s index.php to access site A’s theme folder? Right now, Site B can only access Site A’s system folder.
Mod Edit: Moved to the Technical Support forum.