NOTE: This issue has been resolved in the 2019-05-21 cumulative update (KB4497934). More information here.
Upon restoring a Virtual Machine that was backed up from a Windows Server 2012 R2 host to a Windows Server 2019, you might encounter a problem booting this VM.
If you take a look at the Hyper-V Worker event log (Event Viewer → Application and Services Logs → Microsoft → Windows → Hyper-V-Worker→ Admin), you'll find the following event:
Critical 09/11/2019 15:11:36 Hyper-V-Worker 18560 None
‘Exchange’ was reset because an unrecoverable error occurred on a virtual processor that caused a triple fault. If the problem persists, contact Product Support. (Virtual machine ID 1CD13F50-5E38-4034-B1E8-5D7342BBDD03)
This is a bug identified in earlier versions of Windows Server 2019. Specifically – the bug is exposed by the IPv6 boot data that is stored in the firmware of a Generation 2 virtual machine. Note, this will not effect Generation 1 virtual machines.
Update your Windows Server 2019 to the most recent patches available, specifically KB4497934, which will address your issue.
A workaround and more information can be found in Ben Armstrong's blog here.