If you are working with System Center Virtual Machine Manager (VMM) and trying to configure Logical Networks on a Hyper-V host, here is an issue you need to be aware of.
If the display name of the network adapters on your host contain the square bracket characters (Eg. [ or ]) then the creation of the Logical Network on the host will fail with a rather spurious error message. Check the display name of all of the adapters on the host and ensure that they do not contain the square bracket characters before you go through any other troubleshooting. You could save yourself an hour or two.