My colleague found the answer to the problem at http://communities.vmware.com/thread/146728
I did a "service mgmt-vmware restart" on the hosts having the issue and this cleared it up.
Confirmed, in my case that solved this issue.
Before restarting the service i get this two errors when i tried to migrate VMs:
"A general system error occurred: Invalid fault"
"An error occurred while communicating with the remote host"
Greetings