Starting in 6.0.3 and 6.5 you can disable NSD in the Server Document/Basics Tab/Fault Recovery section, but that will not prevent your crash. It will keep NSD from running after the crash and collecting information about the cause of the crash.
It sounds like a situation that used to generate a message in R5 now causes a crash in R6. You should open a customer support call on this. They will want the NSD log file.