Hi guys,
I had the same issue when I tried to convert a couple of VMs from a VMware Server 2.0.2 (backuped on USB-HDD) to an ESXi 5.5 with my Windows 7 PC. Beside all hints, my solution wasn't the Admin rights, or older versions or even certificates. It was quite simple but it tooks a bit to get it.
In my situation a defrag of the vmdks with the build in tools of VMware Workstation helped to let the converter work. It seems the error occured because the converter couldn't read the vmdks correctly.
Cheers,
Chris