Console ran fine, not GUI, possibly because of environment, I just ran it from lib,
H:\tmp\qt6\lib>helloworldconsole.exe
Hello, world!
H:\tmp\qt6\lib>helloworldgui.exe
qt.qpa.plugin: Could not find the Qt platform plugin "os2" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.
Killed by SIGABRT
pid=0x8ebd ppid=0x005a tid=0x0001 slot=0x00b5 pri=0x0200 mc=0x0001 ps=0x0010
H:\TMP\QT6\LIB\HELLOWORLDGUI.EXE
Creating 8EBD_01.TRP
Moved 8ebd_01.TRP to W:\var\log\app\63780735-8ebd_01-HELLOWORLDGUI-exceptq.txt
Edit: setting PATH and BEGINLIBPATH resulted in the same, something is broken when it comes to finding the plugins, or perhaps loading them, which I assume use DosLoadModule()