Problems with su´d users in X (KDE2?)

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Hi out there!

I have a small but nasty problem when running KDE2.
If I try to run a command as another user as the one who is "really" logged 
in i get the following error message:

ERROR: KUniqueApplication: Trying to launch kdeinit.
Xlib: connection to ":0.0" refused by server
Xlib: Client is not authorized to connect to Server
kdeinit: Can't connect to the X Server.
kdeinit: Might not terminate at end of session.
Xlib: connection to ":0.0" refused by server
Xlib: Client is not authorized to connect to Server
kmail: cannot connect to X server :0
DCOP aborting call from 'anonymous-21029' to 'kmail'
ERROR: KUniqueApplication: DCOP communication error! 

I run potatoe on an i386-compatible.

Thanxx a lot

Andreas Maresch