[CA] [CA11] ST70 + SPX30: "Calibration required" warning at startup

[Korrigan]

After 4 years and 30,000 NM of reliable service, I just got this warning message "Calibration required"when starting the autopilot, a week ago.
As I took note of all the settings of the autopilot, I compared the stored values with the ones I had in my notes and they all match (rudder gain, counter rudder, damping, motor phasing, etc.).
Despite the warning message, the autopilot kept its course okay for the 5 NM we had to motor that day, without any error message.
When we reached a protected bay with no wind or wakes, we performed the compass alignment procedure and completed it with 4° of deviation, which is satisfactory. Once again, the pilot steered ok to our next destination but the warning message is still present at each startup.
The heading displayed on the ST70 looks ok, it does not change erratically like when the fluxgate compass is not working fine.

I'm puzzled by this issue and I find myself having now a confidence problem into my autopilot, before heading to a 2000 NM crossing with only 2 adults able to take watches.
Therefore, I'm looking for some advices to perform further troubleshooting.

I'm also wondering if:
1) the warning message I get is only about compass calibration or it could refer to other parameters
2) there is way to read the compass calibration information stored in the autopilot.

thanks in advance