Adding new evaluate license - Invalid license key: license file not found

Hello folks,

I've been using an old version ov vSphere vCenter. I've now upgraded it an want to add my new evaluate license key, since the old one is out of date. When I try to do this by going to Home->Amdinistration->Licensing-><my host>->Change License key, from the vSphere client connected to the vCenter, the following error occurs:

Invalid license key: license file not found.

Any idea what I'm supposed to do?

Information:

vSphere Client version: 5.1.0

vSphere vCenter Server 5.1.0

ESXi 5.1

Thanks in advance.

EDIT: I use the license key which I got when I downloaded the trial. At "My evaluations"-page it says: