Black Mesa VR MOD, Chapter 4, in the Administrative offices (the large suite full of Vortigaunts). I rescued the lady scientist and the guy who was listening to the rescue radio announcement, I went into the room with the ladder up to the ducting above the sofa, I knocked off a couple of panels so I could climb the ladder, killed the crab ... but I cannot jump from the ladder to the ceiling/duct. No yellow landing area appears and if I try to step/jump off, I fall down as soon as I let go of the ladder.

I have replayed this several times (from turning off the turret), and every time, no matter what I do or do not do, I cannot get off of the ladder and into the ducting. Any theories on what I can or should do?