This scenario generally can occur when upgrading an existing ESX/ESXi host to a newer version of ESX/ESXi server on VMware vSphere 4. VMware and ESXi server handles the networks differently within its OS which long story short treats the fact that if the management network and VMkernel networks are on teh same VLAN that it complains whereas in prior versions it ignored this condition. To rid yourself of this error you need to perform two things:

1. Set every ESX/ESXi server to the same exact Virtual Network Name. If you have VMware Distributed Switches this is an easy endeavor. Else, you have to manually set each host or run a script to do so.
2. On the VMware vSphere console, right-click the cluster and in the VMware HA settings go into the Advanced settings and add the following:
   
   Option: das.allowNetwork0
   Value: Service Console
   
   NOTE: The Value for your installation will be whatever you set the management console name is for your environment.
   
   
3. Sage your settings and right-click the host in questions and choose to reconfigure HA and the error should go away!