I encountered a problem rather similar to the OP’s yesterday, and fought my way through to a solution that made it work for me, after all the “obvious” things (installing the patch, disabling IPv6 protocol support on the endpoints, etc.) had been taken taken care of.

I posted the result (disable any UPnP service on your gateway) on reddit in hope of other Freelancers finding that useful: https://www.reddit.com/r/freelancer/comments/7ydtrg/problem_and_solution_flserverexe_freelancer/