waiting for session master to start

I hope someone can help me with this problem?
This is our configuration:

Netware 6.5 with zenworks 7 SP1 (Image server)
We use a seperate tree (IMGPARK) for the imaging but it's in the same
network. The other tree is a cluster environment and has no connection
with the IMGPARK tree.
This network is a B-Class 192.168.XXX.XXX and the WS's get there adresses
from one of the two Windows AD DHCP servers.

Now the strange thing is when I installed the products it was working
fine. I connected to the other (IMGPARK) tree with consoleone, made a
session on the server object in the tree. I walked to the WS started it
with PXE and it booted into the session. It saw the session and started
nicely like I want. And then the black day came. It saw the session I
started, but the WS comes with the message "Waiting for the session master
to start"???? and never goes a way. I throwed away an object by consoleone
like mentioned in a previous message and it started again, but now it
stoped again. I thought maybe a bug and I installed SP1 for zenworks 7 and
that worked for one time :-( I installed the server about tree times now.
I don't think that's a good solution ;-)
If you familiar with this problem I would like to hear how you passed this
problem. I'm getting a little sad of this one :-(

regards,

Maarten