Are you doing this while the existing image is online or offline?

Are you choosing to import a physical server or a stand alone virtual server?

I would try switching one or both of the options above and see what happens....

It's a vm running on virtual server (.vmx file). VM is offline.

Can you post the error logs?

You can always try to copy the VM via SFTP between 2 ESX servers and this would work if they are not in the same VC

If they are in the same VC, why not try and clone the VM by rightclicking and press Clone ?

You could always try to run a physical to virtual conversion, this is what i usualy do when i can not afford to have the VM shut down to long

Good luck