I've had this error lately while logging into my vCenter server using powershell (connect-viserver).
Could you give some more background information ?
VITK version, from where do you run the Connect-ViServer, version of your VC...
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also, make sure you dont have a .net patch that was borked... that happend to me, ended up re-applying .net patch to fix
VITK version is 1.5.
Im running the connect-viserver from my workstation and the version of VC is 3.5 update 2.
My diagnosis so far is that it is a problem with VC. Once the VC is rebooted(the service restarted and the machine rebooted). I can use the connect-viserver command correctly again.
Ive also tested it by running the command via other machines and it does the same thing there, so it is unlikely to be a problem with my workstation.
Add the VC server IP Address in the exception list of proxy and then try to connect.