It is.. I can't make heads or tails of it. Does it give you linear numbers? Like 6001, 6002. That is, it doesn't skip numbers, like 6001, 6003, 6007 etc.
I'm thinking it must have something to do with permissions and that you are within a domain. That being "admin" isn't truly admin, as your user might still be restricted to a certain set of rules.
I'm not familiar enough with domains to know for sure, but if you have anyone you could ask, ask if there are any port-restrictions. Especially at 6001 and above. And if you could try temporarily allowing those ports to see whether you still experience the problem.
The really good news is that we've narrowed it down into a reproducible problem. I think we're close to solving it once and for all.