remote login with XDMCP from Login-Manager (kdm)

Since 12.1 I can’t login to other machines from the login window (standard kdm without theme) via XDMCP (with Alt-R).
The window opens and shows only the local host.
Keyboard input is ignored, the mouse can be moved, but as soon as it touches the window it vanishes and the local login window reappears (where kbd and mouse work properly).
In 11.2, 11.3 and 11.4 everything worked as expected and I didn’t change anything in installation routine.

How can this be fixed?
Did I miss any changes here from 11.x to 12.1 ?