WSUS 3.0 MMC Issues

Hello,

I'm racking my brain about this one for quite some time.

We are running WSUS 3.0 and everything works good so far except for the MMC. 
On some Computers the WSUS Console refuses connection to the WSUS Server - on 
others the connection works ok.
This happend suddenly - one day I approved some updates using the MMC on the 
affected Windows XP System - the next I couldn't connect to the Server 
anymore. It keeps telling me the WSUS Server could be using another port, 
wich is apparently not the case as moving to another system (same username of 
course) works with the standard port.

What I tried:
I deleted the %Appdat%\Microsoft\MMC\wsus file
I uninstalled and reinstalled MMC 3.0 and the WSUS Snap In...

Nothing good came form it so far.

Up until very recently I worked around the problem by using the MMC on my 
secondary machine - but now I get the same problem there :(

Is there help?