When you disconnect the red lead, you no longer have any power going to the RX. Hook a standard RX battery to the RX and it will (or ought to..) work.

EDIT: When I typed that reply, I was not aware that this controller might be an OPTO coupled controller. I saw a reply from another poster regarding this and they were pretty sure it was. I have no experience with OPTO contollers, the reply I gave was based on plain ol' ESC/BEC combinations where if you disable the BEC via disconnecting the red BEC wire, you must supply another source of power to the RX.

And I don't think overvoltage to the RX is why you can't run the BEC on 22V, it's that the BEC delivers less power as voltage increases, so at 22V there is just not enough power from the BEC to run anything..