Haven’t tried the RME multi client drivers, but possibly it’s because both Cubase and Wavelab are addressing the same output channels?
Although in that thread they say RME addressed that.
Maybe get later drivers if you don’t have them, although I have 3-4 year old drivers that don’t seem to have any problems with start up delay in Wavelab 8, and not updating because of some reports of delay with current drivers.