After much trying and checking other machines for cross-referenct it boiled down to Import-Module Hyper-V not working on my machine, while at the same time Get-VMSwitch would load the Hyper-V module.
This brought me eventually to $env:PSModulePath, which on two (!) of my machines looked like this:
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(notice the System.String[] at the end) while on newly installed machines it contained %SystemRoot%\system32\WindowsPowerShell\v1.0\Modules (and not the System.String[] rubble)
So. Changing PSModulePath environment to look like this:

fixed the whole problem.