Bei einem Upgrade von Hyper-V Server 2012R2 nach Server 2019 verlieren die virtuellen Switches ihre gültige Konfiguration, sodass keine Netzwerkkommunikation mehr möglich ist.

Folgender Artikel weist auf dieses Problem hin: https://american-boffin.com/2019/01/17/virtual-machines-do-not-boot-after-moving-from-windows-server-2012-r2-to-windows-server-2019/

Workaround

In the meantime, it is possible to work around this. To get the virtual machine to boot you need to get Hyper-V to create new firmware entries for the IPv6 boot data. The easiest way to do this is to change the MAC addresses on any network adapters connected to the affected virtual machine. This process is different for virtual machines with dynamic and static MAC addresses.

The following update should resolve the Problem : ​May 21, 2019—KB4497934 (OS Build OS 17763.529)