Simple one this.
Trying to convert a VMWare Server machine to a VMware Infrastructure machine the new ESX4i one using VMWare vCentre Converter standalone and I got the above error – “Session not authenticated”
Did a quick bit of research, and found that in task manager I for some reason had VMWare vCentre converter standalone running twice. I didnt start it twice, but the first time I tried the convert it had failed, so I guess for some reason it started another instance.
Killed both processes off restarted VM Converter and all was happy.