Edwimb,
I've also seen this error but not consistantly and never worked out why, but it was after SVS was installed or after having problems with an SVS layer usually. uninstalling and reinstalling SVS sorted it however i only knew about those problems because of the issues with the SVS Layers we were using.
I also run SVS 2.1, DS6.9 SP4 and NS 7.1.
I don't beleive its just to do with the NS Agent though. I can understand that you are consistantly seeing this when you are installing it.
Have you had a look through the logs to see what is occuring in those? You can't presume the issue is this even though there is some consistancy to it without digging through the logs as there are multiple agents and plugins that could possibly causing this effect.
I would recommend installing just the Aclient and NS Agent on a fresh build as part of the build and see what happens on that to see if its just happening on established machines.
I definitely recommend getting the log files from the install of NS and also the aclient logs from the time that you are installing it up until the error occurs and slowly go through them to see if anything stands out.
Also try uninstalling the SVS and NS agent, restart and just install the NS agent and see if you get the error then.
i know its a fair bit of digging about but hopefully we can help you work this out.
Jim