Are your hosts all in the same cluster or in different ones? It's likely an EVC mode incompatibility and the NX/XD flags on the VM are not compatible with the destination host.
If you are stuck, shutting down vCenter really shouldn't be a very big deal. Just note which host it is on and use the client to connect to it directly. Shut it down, and mark the CPUID Mask to hide the NX/XD flag. Power it back on, and when the vCenter service is back up, you should be able to migrate it. The whole process shouldn't take more than 10 minutes.
Out of curiosity, is your vCenter the appliance or a Windows server?