.NET SDK cannot be found after installation

[sasaendler]

I was trying to get the .net intreactive notebooks running on a (Win10) PC that has no dev IDEs installed.
We installed VS Code + .Net SDK 7 and the Polyglot extension.
Still at every start a message indicates that the SDK is missing.
"dotnet --list-runtimes" lists the recently installed SDK.
Un- and installing the SDK didn't help. VS Code was closed at installation time - this seemed to be a bug in the past.

Can you give me a hint? What am I missing?