HyperV fails to run VMs that are connected to a virtual switch, but works for VMs that are not connected

I am running Windows 10 20H2 on a HP EliteBook G4.

I have the following problem:

I cannot run VMs attached to a virtual switch.

• All VMs run successfully when not using a Virtual Switch ("not connected" under network adapter)
• When I assign a virtual switch (any type, any kind) to a VM, it hangs in starting at 10% and eventually fails with a timeout error. I tried both an empty VM not containing an OS (this should at least show the UEFI screen) and a VM containing an installation of Ubuntu 20.04 LTS.

I expect that I can run VMs connected to virtual switches.

What have I tried?

• Disable Antivirus
• Network reset in Windows 10 (multiple times, e.g. netcfg -d)
• Uninstall Hyper-V, delete any superfluous networking interfaces and reinstall it
• Upgrade Windows 10 (it was at 1903 before that)
• Update ethernet driver with that from HP
• Delete ethernet driver and use the default
• Upgrade BIOS
• file system and system files checks

In total I already spent about 20h also involving people from IT on this problem researching and trying various commands, following various websites. It is a problem I have not found so far (or still missed?). I hope to find someone that had the exact same problem and can remember the solution.

What errors do I get?

I think these are the relevant ones:

• Hyper-V-Worker
  • Event ID 12006 with following message
    • 'Ubuntu20.04.1LTS' Synthetic Ethernet Port: Failed to finish reserving resources with Error 'This operation returned because the timeout period expired.' (0x800705B4). (Virtual machine ID XXXX-XXX-...)
  • Event ID 12030 with following message
    • 'Ubuntu20.04.1LTS' failed to start. (Virtual machine ID XXXX-XXX-...)
  • Event ID 12010 with following message for a "Generation 1" VM with a legacy network adapter
    • 'Emptyv1_net' Emulated Ethernet Port (Instance ID XXXX-XXX-...): Failed to Power on with Error 'Access is denied.' (0x80070005). (Virtual machine ID XXXX-XXX-...)
• Hyper-V-SynthNic
  • Event ID 12670 with following messages
    • 'Ubuntu20.04.1LTS' failed to allocate resources while connecting to a virtual network: This operation returned because the timeout period expired. (0x800705B4) (Virtual Machine ID XXXX-XXX-...). The Ethernet switch may not exist.
    • 'Ubuntu20.04.1LTS' failed to allocate resources while connecting to a virtual network: Access is denied. (0x80070005) (Virtual Machine ID XXXX-XXX-...). The Ethernet switch may not exist.

As the Repair Install of Windows 10 with an In-place Upgrade to Windows 20H2 did not help, it seems like you need to reinstall, unfortunately.

I suggest to backup your data and reinstall Windows from scratch, meaning not as an upgrade. You will need to reinstall all your applications.

Reformatting the install partition is optional, and perhaps should only be used if the simple installation failed to solve the problem.