The RSL relies on a specific differential voltage being present between the L1 and L2 signal lines in order to detect clock pulses and data.
It also relies on proper power getting to each Remote Station (the "RS" boards). If you have a lot of current draw on those power wires you can end up with different "reference" voltages (the common 0V) at each Remote Station. Remember Ohm's Law, V=IR ! This is typically a problem with very high rises or where you have some power-hungry fixtures attached to the Remote Station power, or perhaps a bad connection. The original AMP insulation displacement "IDC" connectors were notoriously unreliable if you didn't use AMP's special tool and tried to push the wires into the "V" shaped insulation piercing pieces.
And there's also proper Line Termination. Depending on whether you have "Master In the Middle" (e.g. LCB or LCB II with a single RSL) or "Master At The End" (RCB or RCB II and most later GCS-based systems with separate Car, Hall and Group RSLs), you need to be using the right terminator board.
Lastly, there's also the issue of proper RS board addressing. If you have duplicate boards you can get parity errors if the active inputs are different between the two boards. Have you verified that all the RS boards in the system are correctly addressed?
It sounds like that one board on floor 3 is, as you say, dragging down the entire RSL. The real question is what is so "special" about that one board? If you disconnected the buttons from it, does it still drag down the entire RSL? If you move that board to a different floor, or put a brand new RS board onto floor 3, does that change anything?
I suspect that when you've got the hoistway RSL connector 1/4 of the way into the LCB II that you are possibly not powering up that segment of the RSL properly, or maybe the L1 and L2 signals are not fully connected in some weird way. Unless you run the Self Test and look at which missing RS boards are there, you don't really know if the hall button RS board for floor 3 (or for some other floor) is communicating or not.
I do not understand your comment about changing RS boards on "roof 16 and 17", which maybe is a typo for "floor 16 and 17". Without knowing EXACTLY how many floors you have, and what other RS boards are connected it's a bit hard to say for sure. Perhaps you can share more details about ALL the boards on the RSL.
One more thing - are these "genuine" RS boards? There was a time when Chinese-produced Otis controllers were being shipped with locally-made "clones" of the RS boards. Some of them even had similar part numbers as the German-made ones, but they were prone to all sorts of weird failures, including bringing down the entire RSL bus.
