You can connect a Multiplus to a GX device through a mk3 to USB, bit any Multiplus connected like this is viewed as a secondary system. The primary system MUST be connected directly by VE Bus cable to the VE Bus port. If connected through a mk3 to USB the Multiplus may not take part in all system calcs/controls although recent firmware updates prior to 3.66 bring the secondary Multiplus under DVCC control.controls
@remcohn I do not know why you have an error with the direct connection, I do not know about 3 phase or parallel systems. I suggest you solve this error and connect your set up as the primary Multiplus with a direct VE Bus connection like it should be. Make a new post about your error and see if anyone can help.