hi joerg_turla,
first you need to view system requirements for qt rundtime sdk
Qt SDK for Windows version 5.4.1 minimum (msvc2013, OpenGL) See important note below.
To access HTTPS or SSL-enabled web services on Windows, the Qt SDK requires the latest SSL/OpenSSL libraries be installed. See your operating system documentation for more information.
Microsoft Visual Studio 2013 C++ compiler
Microsoft XML Core Services (MSXML) 6.0
Microsoft .NET Framework 4.0
Microsoft .NET Framework 4.5
Qt 5.4.1 need to be version (msvc2013, OpenGL)
after this you install ArcGIS runtime sdk for qt
and run postinstall when you specify qtcreator.exe and qmake.exe
then run qt creator for example: open new project - > ArcGIS - > one choice one from 2 both templates and go qmake and run.
if your Qt creator is with mingw or version is under 5.4.1 its normaly to have this issue