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 I've now upgraded it an want to add my new evaluate license key, since the old one is out of date.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 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.