I think the connection between autopilot and yaw damper was the eventual fix, but when?
There is no connection between the autopilot and the yaw damper on the 747-400.
They are completely independent.
The Yaw Damper System uses inputs from the Inertial Reference Units (IRUs) and Air Data Computers (ADCs) and 4 dedicated accelerometers (positioned at the front and rear of the fuselage).
The IRUs and ADCs send data to Yaw Damper Modules (computers). The Yaw Damper Modules, using this data, compute and then output signals to the rudder actuators for coordinated turns and general yaw damping.
The dedicated ("modal") accelerometers sense fuselage twisting and the Yaw Damper Modules use this information to compute and send minor corrections to the rudder actuators to try to dampen the twisting.
Rgds
NSEU