Getting ServerFaultCode exception when trying to get refrance to a VM

Hi.

I am using PHP 5 and its builtin SoapClient to connect to vCenter 5.5 web service. Login and log out functions are working but when call any FindBy methods I get the following exception:

SoapFault exception: [ServerFaultCode] The object has already been deleted or has not been completely created

I suspect this is not an issue with parameters but is a general issue with method calls. Please assist me to pass this part of my development.

Thanks in advance.