Furuno Autopilot Rudder Drive Error

[Howler]

We have two completely redundant autopilot systems (Furuno electronics and L&S hydraulics) on a 50 ft monohull. Configuration described below. Each is intended to be used alone, with the other as a backup. Autopilot 1 operates on the port rudder and Autopilot 2 on the starboard rudder. There's a tie rod to keep the rudders in sync.

After 10-20 minutes of use, we get a "Rudder Drive" error and the system stops working. After powering down Autopilot 1 and starting Autopilot 2, we get the same error!

• Electrical/data connections seem OK.
• There’s no evidence of a hydraulic fluid leak and the reservoirs are full.
• The systems work fine until the error occurs.

This is baffling as the systems are truly independent as far upstream as the processor units and control heads, meaning it would be an unlikely coincidence if both systems have the same issue. The error is supposedly raised when the rudder position sensors detect less than 1.5º of movement following a turn command.

Having just acquired the vessel we have limited experience with it. The prior owner insists it worked fine. The only thing that changed as far as I know is an update to chart-plotter software to which the autopilots are networked.

Attached is information on the configuration and some hypotheses, but it's a shot in the dark. Any experience / wisdom to share on how to pinpoint this?