Finally discovered the error here. Inspired here:
https://forum.qt.io/topic/39319/solved-qprinterinfo-availableprinters-don-t-list-available-printers/2
the .dll file was from the previous version before upgrade and somehow didnt get upgraded along. So copying the file manually to the location instead of the previous dll version made it.
Thanks for help,
Kosmic

@Qjay
This may not help because I do not know quite how you would do this directly from Qt, but when we want to print a PDF under Windows we use the underlying Win32 function ShellExecute() (https://msdn.microsoft.com/en-us/library/windows/desktop/bb762153(v=vs.85).aspx) to go (effectively) ShellExecute("print", native-path-to-file), and let Windows shell sort it out.

This is effectively the same as right-clicking on the file and picking the Print action. So you don't need to know anything about the actual process which does the printing, its arguments etc.

I am stupid... I included another project file that requires printsupport as module. Since it doesnt exist for Windows Phone/RT the error is shown.

So this is solved...

Thanks anyway