thanks for the feedback.
I have 3 machines which are configured for programming. I thought I have the issue on all three of them.
I just tried it another way. Installing the reference through the extensions and update.
Then new project =>MVVMLight win10
But then, I have the same problem.
The System-references are gone?
EDIT: just tried extra things:
New -> Blank app (Universal Windows).
These are current references:
After testing with Nuget Prism MVVM, all other references are kept:
Then removed Prism, situation is again as Situation 1.
through Nuget, added the MVVMLightLibs and all other Nuget packages are gone!
Nuget package manager however, still find them, but my solution doesn't...