Of course I installed the RME drivers, but apparently Audition v2 and v3 force the RME Digi96/8 digital output in ADAT mode.
See first message in this thread by Thomas W. Bethel.
Yes, I read all that ages ago. You will note though that I said a properly working ASIO driver. Since I'm not aware of this happening with any other sound device by any other manufacturer, then despite what is claimed, I still think it's RME's fault.