Ok, all possibilities shall be tried. So I tried to install SnapCenter_Windows_vSphere_Plugin direkly at the SCV server, and it worked. Then I restarted install from SnapCenter, got a fault, warning, choose next anyway and the job resulted in a OK status. Still fault at the vCenter, så I rebooted that one, and no is everyting OK, big smile.
I hope this can be useful for anyone else ;D
Have a nice weekend ;D