Hi Noa,
I'm also going to contact TDT.
The BNC output channels are driven by a logic isolator IC. It's a fairly unique chip - it contains a tiny (albeit inefficient) isolated DC/DC converter in addition to the standard logic isolation circuitry, to provide power on the far side of the galvanic isolation barrier. When I speak with their engineers, I'll try to find out why it has trouble driving whatever is on their end.
While we're resolving the issue, one thing I'd try is triggering via the 3.3V LED channel on a free behavior port. You'd connect it to TDT via the LED + and - terminals on a port interface board and wire to BNC adapter. Then, in your state matrix (assuming behavior port 4) you'd use: {'LED4', 1} and {'LED4', 0} to make it high (3.3V) and low (0V).
If it turns out neither option works, we'll find out what it takes to trigger their devices and push out a new Bpod module.
To help the TDT engineers understand what we're trying to do, can you please post the exact channel you're connecting to on your RZ5P device?
Thanks, and sorry for all the trouble!
-Josh