ok I found the problem...
the Cube Serial inteface are driven by TXS0108E Level Shifter powered both side with 3.3 volt making it marginal for the Voxl Primary Serial interface. Switching to secondary interface on the Voxl made it work correctly.

I sent a request for a change in the WIKI to reflect that configuration

40f28124-b098-4482-a038-25c450b9df6c-image.png

@ryan-langlois1 We will definitely continue to support PX4 on VOXL 2. But we are also very interested in getting a port of ArduPilot going on VOXL 2 and have had some preliminary meetings with the ArduPilot community to start the process. But we cannot offer a timeline at this point.