It's likely that this line in your VMMARK2.CONFIG:
Standby/SERVERS="Standby0"
has something unexpected or erroneous. Does it say "standby0/SERVERS"?
If this isn't the problem, compare your current VMMARK2.CONFIG file to your old 2.5 VMMARK2.CONFIG file. Make sure all instances of "standby" or "standby0" match in their format.