But if hes has the CAN bus connected to the empeg's dimmer line

That's 100% sure *NOT* the problem

I've tested some more:
When the riocar is off and then switched on the button lights stay off (the level in the menu shows [MAX] )
If I put the riocar in standby (press top button 2sec) and then back on the button lights work well....
This all with the dimmer line connected to ground, +12V or nothing

Looks like my button lights don't like cold starts...