During an upgrade at a customer site we removed a Dell Appliance that was connected to vCenter. We dit remove the aplliance just by powering it off and then remove it from the entry. Normally you should do an unregister from in the Dell Appliance.
Now we saw that there was a plugin that could not be loaded.
After some research on the internet I passed by on the website of William Lam (virtuallyGhetto).
He has written a nice article that describes step by step how to fix the problem we had. Look here.
Problem solved, thanks William for the good work.