Quixel SUITE windows behind Photoshop window

I have a problem. The Quixel suite window is behind the Photoshop CC2017 window.

I'm aware that this is a known issue, but the solutions I found using Google did not work for me so far.

Quixel seems to have problems to conncect to Photoshop. I already restarted Photoshop in Admin mode, closed it down again. Quixel is still reliably behind the Photoshop window. I uninstalled and re-installed Quixel to a different folder. No effect.

When I click 'Reboot PS' in the Quixel drop-down menu, I get the message 'Successfully rebooted. All memory is now cleaned'. The Quixel window is on top of the Photoshop window, but when I click on any Photoshop UI element, the Quixel window disappears again and is behind the PS window. 

Starting Photoshop prior to starting Quixel has no effect.

Setting

Any suggestions?

EDIT:
Without any apparent reason it started working suddenly... for 1 PS session. Closed down PS just to test it, now the Quixel window is behind the PS window again. Wearisome...