Hi Logtech,
bollocks. Another question: distance of the SFP+ cable? I see that 7 meter cables are supported by EX3400 but the OneX only supports up to 5 meters.
If the cable is only 5 meters, please test the cable between two ports in the EX4300 VC to ensure cable is actually operating as expected.
If you are still having issue, may I suggest you look into buying 3rd party cable where one end is coded for Cisco and the other for Juniper. I think that eg. Prolabs can help you with this kind of cable.