Hi Gert,
I dont think it has to do with running the tests in parallels as you can run multiple projects on an HLK controller at the same time.
I would recommend going through the ARM64 installation steps highlighted in the following doc and making sure that all steps are followed
https://learn.microsoft.com/en-us/windows-hardware/test/hlk/getstarted/step-2--install-client-on-the-test-system-s-
Thanks
Fidaab (MSFT)