Ride height too high on one side, and drops on the other
 

I have a problem on my 2010 F11 that's been bugging me for a year now. First off, just to be clear: there is nothing wrong with the air bags (also known as the bellows). There are no leaks, rips or tears on either the Rear-Left (RL) or Rear-Right (RR). I have had the RL bellows fail before so I have experience of what it's like for one to fail.

So, the rear-right (RR) bellows had deflated completely leaving it lop-sided. I had inflated it fully using ISTA, and the car can stay like this for weeks on end without driving it (only use the car once or twice a month, sometimes less), so it can be on the driveway looking all pumped up. However, when driven, and parked up, it can then deflate completely again. When the car is started, the compressor inflates it back again. I drive, then park up, and then deflates. Now it can be random when it chooses to deflate, sometimes it doesn't, other days it does.

Sometime, though this is becoming more common, is that it doesn't inflate at all, or it deflates during driving!! The only way of getting home safely is to plug ISTA in and manually inflating the bellows, or deleting Transport Mode (if this works).

Having read some articles, I deleted Transport Mode, as I am led to believe that enabling Transport Mode increases ride-height to aid transportation onto lorries, etc, and perhaps it's confusing the EHC (Electronic Height Control) module. This is hit-and-miss and sometimes it pumps it back up, other times it doesn't. But worryingly now, the RL is now almost fully inflated so it looks like an X5! And if the RL is deflated, the car is seriously lop-sided! Using ISTA,

I am able to re-inflate RL to balance it out, but then the whole headache cycle can start again.

ISTA does recognise all modules (Electronic Height Control, Integrated Chassis Module) as working correctly, both Ride-height sensors are working with the correct voltage etc, can respond to all activation tests to deflate and inflate the bellows, so there is no problem there. I've also tried resetting EHC, ICM, DSC, etc but to no avail.

I have tried to adjust the ride height itself, but my version of ISTA errors when I try to save the wheel-arch to wheel flange measurements - the error box

appears with no message showing - INPA does the same. I feel that this is what I need to do but this error is holding me back.

So the questions I have are:
1) has anyone experienced anything like this on this car, or any similar car with air suspension?
2) What causes this?
3) How to correct?
4) Is my version of ISTA/INPA wrong for this job and why does mine error when I enter the measurements? Does anyone have 19" wheels with 244/40/19 profiles and can measure the wheel-arch to "edge of wheel rim" at the lowest point for me for reference?

Thanks for reading