Thanks guys for reading the post and trying to help.
Ultimately the problem was as very often between keyboard and chair. Other incredients were the most confusing error messages of the Android sdk compiler and linker. A part played also a creator bug for setting additional libraries used for building apk archive.
@J.Hilk you could be right with androiddeployqt and also the advice of @SeeLook might helped. However, around that time I made yesterday my final attempt which succeeded and I called it a day.
I think it is not conclusive, but may be the origin of my problems.
The main project folder was initially holding the pro file and everythink was already in subfolders. This is also true for the main sources and java stuff including AndroidManifest.xml. Now by changing to a subdirs project for the building. I replaced the pro in main folder with a subdirs pro file. This requires an additional subfolder for the new pro file due to the subdirs template mechanics. I had added the subfolder and redirected from there to pull the required files and setting the pathes to the other subfolder where the stuff was. I am pretty sure that the pathes were all correct, but apparently not a good idea for the use of one of the tools involved.
After correcting and having a straight increasing cascade of subfolders I managed to have sufficient cinfusion created to keep me busy.
My conclusion is to keep the project setup straight and avoid parallel back-stepping in your overall subfolder structure.
The output of Android SDK compiler, linker and also at run-time were pointing into different directions and always away from where the problem was.