Having a quick google it could be a leak at the front with the most common culprit being the airbags leaking that i have seen.
The ECU will know how much air is going to each bag and therefore how much that corner should lift which it detects from the height sensor.
If it's taking too much effort to raise the vehicle it will fault out to stop the rest of the system self destructing by trying to keep up.
Have a read of the competition below to see if this sounds familiar.
Do you get any drop in height when it's parked up?
2003 RR HSE Air Suspension Problems