Thanks a lot for your responds. Very appreciated.
I have read both KBs 2030250 or 1030719.
On my Datastores I have lot of space. > 300 GB.
But on my ESXi1 and ESXi2 hosts (cluster) it looks like this: 496,00 MB free out of 512,00 MB <-- Can this be the culprit? I have been testing most of the memory settings in vCloud/vCenter, and getting this error still. Very annoying. :-) Running the ESXi hosts on 8 GB USB sticks.