vCenter Wild VM list is empty

I have 3 vCenter clusters added as hosts.
For 2 of them I can see the list of Wilds VM.
For the 3rd one, the VM list is empty, the GUI shows “There is no data available”
It used to work a few days ago. I tried to reboot OpenNebula but still the VM list is empty.

Where do I start with troubleshooting ? Why it suddenly stopped working ?