A small update, as I figured out the misfire might be caused by CAM-Sync issue, as reported here:
It seems, taking into consideration the above, that only the problem with communication remains (red LED on USB2CAN, and messages in Light Client). No idea if this might be HW or V3 SW related.
I did power up the light client with nothing but 2x 120 Ohm resistors at the ends of the bus, speed set by hand (as there are no devices on the bus). Seems fine to me.
And also with Black V3 and CAN stream enabled (the values in the All Frames area are alive in that case and change nicely).
When I disable CAN stream for Black, bus remains auto negotiated at 1Mbps, status is OK and nothing is displayed.
I expect this fully answers the question if USB 2 CAN module might be faulty and leaves only the Lambda 2 CAN / Black integration to be sorted out.
Thank you!