Unable to add files to a newly created Qt project

I create a fresh solution with a Qt Class Library project.

I try to add a simple header file ("VS: Add new item").

I get an error message box stating: "The item type 'None' is not supported by this project item provider. Parameter name: itemType"

I retry to add the header file again ("VS: Add new item").

I get an error message box stating "A file with the name D:\tmpVS2019_issue\QtClassLibrary1\Header2.h already exists on disk."

I try to add the existing file from disk ("VS: Add existing item")

I get an error message box stating: "The desired name for <myFileName> is invalid". After clicking ok a new message box appears stating "The operation could not be completed. Falscher Parameter"

Altogether it's not possible to create a fresh Qt project an work with it! I need a solution or workaround!

Thank you for your efford.