Hi, did you solve your gearbox fault issue? I have a similar problem (plus other error messages and my air suspension not working) and suspect it's a ground issue or a short to ground. So far I've replaced the battery, checked the alternator is charging (13.7V), changed the battery leads, cleaned all the engine bay earths and the earths around the BeCM and the footwell earths. Now I can drive the car an only occasionally get the transmission fault code error when driving under low electrical load but at night at reasonable electrical load if I flick on the brights (sorry) I get the gearbox error every time - same if I use the brakes.
Next steps - check the gearbox connectors near the handbrake, recheck the round multi pin connector under the expansion tank. Check the gearbox loom under the car.
This investigation started because my EAS stopped working. I found after some time that the EAS was actually fault free and what was happening was that when the EAS started the compressor to raise the suspension my poor earth issue (assumed) then caused the height sensor voltage (and hence EAS interpreted position) to change due to the electrical load. The incorrect height (too high) would then cause the EAS to switch off the compressor, load then drops, sensors read correctly and pump back on - end result a cycling compressor and weird alternating sensor heights! Anyone had this? But I can't find the cause! HELP!!!