I am seeing the same problem.
I have a 4 node All Flash VSAN cluster that I am trying to benchmark. When my hosts are running 6.0 Update 2 the io analyzer web front end has no problem adding them to the configuration and running workloads on them.
When I upgrade these same hosts to 6.5 RC, the web front end can no longer log in, even though the underlying VM can ssh to each host without issue.
Will VMware be updating IO Analyzer to work with 6.5?