Hyper-V Virtual Ethernet Adapter not working probperly - Code 31 on Host

I have a Windows 8.1 machine with Hyper-V installed. From a "clean" Hyper-V configuration I open Virtual Switch Manager, create an External switch with "Allow management operating system to share this network adapter", click OK. Everything seems to work properly but the host loses network connectivity.

Opening Device Manager, the "Hyper-V Virtual Ethernet Adapter" is shown under the Network Adapter section with a yellow triangle, and the following message:

This device is not working properly because Windows cannot load the drivers required for this device. (Code 31)

An object ID was not found in the file.

I tried "Uninstalling" the device, seems to work, but the device doesn't actually remove. I tried removing and re-adding the Hyper-V feature completely, no difference. Tried scf /scannow, no problems.

System and Application logs show no errors. The Hyper-V-VMMS > Networking log shows the following:

Log Name:      Microsoft-Windows-Hyper-V-VMMS-Networking
Source:        Microsoft-Windows-Hyper-V-VMMS
Date:          10/24/2013 10:53:07 AM
Event ID:      26088
Description:
Failed to apply static IP settings to internal Ethernet adapter {A813DE9A-BE70-4FAE-AD31-BE4D54505A4B} ('885435B8-BE65-4EE9-826D-AB56035237ED'): Unspecified error (0x80004005).

If I try to remove the Virtual Switch in Hyper-V Virtual Switch Manager I get this in that same log:

Log Name:      Microsoft-Windows-Hyper-V-VMMS-Networking
Source:        Microsoft-Windows-Hyper-V-VMMS
Date:          10/24/2013 11:19:47 AM
Event ID:      26142
Description:
Failed while removing virtual Ethernet switch.

Trying to remove the Virtual Switch leads to an error:

Error applying Virtual Switch Properties changes

Failed while removing virtual Ethernet switch.

VM Networking does work.

Apparently when I uninstalled Virtual PC it left the virtual network devices installed in Windows, but without the driver software (no idea how that's possible). These are known to conflict with Hyper-V (though I would expect some kind of sensible error). Removing the devices in device manager, then removing and re-adding the Hyper-V feature cleared up the errors.