I was trying to use vCenter Converter and kept getting this error when trying to log in. I then realized I was doing it wrong. I didn't need to login. This worked for me, but it may or may not work for you.
Launch vCenter Converter Standalone -> select "Connect to a Local Server"
Click "Convert Machine" -> Select your source
Click Next - > Select "VMware Infrastructure Virtual Machine" -> Browse to your file
Enter your ESX/ESXi server details -> Type in your username/password (Remember if your using active directory authentication its domain\username)
Click Next -> Type the name that you want it to show up as in vSphere -> Click Next
Choose your destination (which pool) -> Click Next
Edit any info of the Virtual Machine if you need to -> Click Next -> Verify everything -> Click Finish