I made some additional tests:
One JoyWarrior controller either USB2 or USB3 works. Both controllers @2+3: continuously 2 beeps per second, no boot (the chips are normally used in joysticks, here they are used to register switch positions).
USB HUB (EXSYS "EX-1177HMV") @ internal / USB Bracket: List of boot devices appears but no keyboard available -> no boot. (no bootable device is attached to the HUB!)
Everything connected: Beeps + bootlist with no Keyboard...
Also to be stated:
Everything works well if attached after boot (so nothing seems to be defect)
Both JoyWarrior chips have been attached previously at a Tyan Toledo T35q without any problems.
The RMM3 is not yet delivered.