Thank you very much for your explanation Mike.
You guess that it is my printerdriver causing the problem.
If that is the case then please explain why it has worked perfectly for 7 years.
Its the same printer, the same driver (there is no other driver) that previously worked just fine.
This is the reason why it is so hard for me to accept that its the driver/printer.
You stole MY question.
The code that prints the page background hasn't changed in years and the changing variable here is the rebuilt PC. We know QU is sending 0,0,0 to the driver because your print-to-tiff gave you true black. When QU prints to file, it runs the exact same code as that which sends the data to the printer: it just reroutes the data to a file. It's more likely that the driver isn't "happy" with something regarding your new PC build.
Mike