I finally found my error. Turns out that there is really nothing wrong with the code itself, but I've somehow destroyed my VisualStudio project. I am really not an expert for these things, but here is what worked for me:
- create a new empty project in Visual Studio
- copy your cpp files (all of them also the *.h files) into the new project folder
- add them to this new project by right clicking the project and using "add New item"
I know this is the straightforward way to do it, but I cannot see how I broke the old project (it should be simple enough after all)
Thanks to all of you - thanks especially to otisonoza and Angew, for setting me on the right track that there is nothing wrong with the code (which works on their end), but with the Visual Studio project.