I checked the "Motor" terminals in the S3G enclosure and there was no corrosion.
I also checked the rudder reference and it appears in good condition. I operated the wheel with the Controller on "Auto" and the rudder reference on the Controller appeared to be in sync with the wheel position so wouldn't that lead you to believe it is functioning normally?
If the "Motor" terminals are not energized, then there is no way for the controller to operated the linear and rotary drives, which lines up with the manual explanation of "Drive Stopped" as "The autopilot is unable to turn the rudder...". It won't be able to turn the rudder without the motors.
I can't think of anything else that would cause the "Motor" terminals to be de-energized when the Controller is ON, in AUTO, and with a command +10.
Unless you, or someone else has an idea, I'll dismount the S3G and send it to your reference in CO.