Hi Plamen,
Within Converter, you have the button called 'convert machine', select that and then select a powered on machine, host name, username and password and whether its Windows or Linux, it's at this point when you click next you get the error. This was from using Converter installed on the vCenter server. A colleague had the same problem running it from the vCenter server and his PC, but when he connected to the Converter client from his PC and didn't specify the vCenter server name, it worked fine and we have our P2V.
cheers,
Bob