The issue has to do with the way to install my application. The application has been installed directly through AirDroid when the message came. After organizing a fitting SD card the issue remained.

A slightly different installation process through copying to the internal memory and installation worked on Android 8, this has been verified with Android 7 and worked too.

The final test to install without additional SD card by copying to internal memory and installing from there does also.

@Mathan-M
Following Java code can be used (Can be easily used from JNI if needed)

// android.os.StatFs StatFs stat = new StatFs(Environment.getExternalStorageDirectory().getPath()); long bytesAvailable = (long)stat.getBlockSize() *(long)stat.getBlockCount();

I have not used Qt on WinRT for a while but as far as I remember direct access (without using a file picker) will not work due to sandboxing. WinRT uses StorageFile objects (instead of FILE stuff known from C) and Qt caches these StorageFile handlers internally but only after using a picker.