from the manual;
When holding a constant course
in STANDBY mode, the heading
The Autopilot is connected to a Raymarine Pathfinder
unit with the “Bridge NMEA Heading” option switched
on. Disable this feature on the Pathfinder unit.
Pretty sure the C series had this also.
And it says No Data or No rudder data?
"No Data" is the usual syntax that can include a few communication issues including rudder.
Resolve your data loop (above) and your problems should go away. Or it's time to call in a tech. Good luck, please let us know what you find.