Hi Arian,
Look in your ADSM server log for the following messages:

ANR0481W Session 10 for node NODE (Netware) terminated - client did not respond
ANR0481W within 300 seconds.

If it is in your log, the 300 seconds timeout is still not enough in your case, 
so increase the COMMTIMEOUT value in your servers option file.
Kindest regards,
Eric van Loon