I've been restarting all my tile0 machines before I do a run just to ensure there's nothing remaining to be cleaned up.
Here's what I've found so far:
Even though stafproc was being loaded at login by the Task Scheduler and it was running with maildomain0\Administrator privileges, after removing from domain, and re-adding I deleted the STAF task and re-created it. This time I gave it elevated permissons.
I also went in (not knowing other STAF dependencies) and elevated the Microsoft Exchange Load Generator Remote Agent, cygserver and RabbitMQ services with maildomain0\Administrator login privileges instead of SYSTEM.
Now I can run the loadgen from primeclient without the error. I am currently re-running after rebooting everything.