I've found the culprit, and the reason is because:
You have used QtCreator with some Qt version before, and now have installed the same Qt version in a different location: There's a bug that in this case the information where to get the help file is not updated.
The solution is simple:
Please try explicitly removing the entries in Tools -> Options -> Help, and either restart QtCreator, or add the qch-files from your Qt version explicitly.
Doing the above procedure returns my Qt Creator help content properly.