SLS inactive: what about a broken accumulator?

[X54L1FE]

Weeks ago the SLS system was checked by the O®iginal Steale®:

- They reprogrammed the EHC
- Checked the sensors
- Calibrated the SLS
- System error check: No errors at all
- Air strut check: no leaks
- Replaced the relais that connects the compressor with the EHC
- Checked the compressor for faults

A few weeks after that, my usual mechanic found more:

- A faulty FR levelling sensor -> replaced both FR and FL
- Two leaks: one in the hose and one on the connection of the air strut

The air struts etc were replaced 4 months ago. But still, the FR kept sagging. The compressor tried to lift FR again, which worked for 5 minutes and so on. And finally the SLS inactive error came up as well. Compressor's lifting capabilities are outstanding: Lifts one side within 2 secs.

And then this happens: Yesterday I drove through the carwash and after hitting the road again, the SLS inactive error was gone! Only the wheels were set quite low still, at more or less 690mm each. So we tried to calibrate the SLS again, with the Snap On Modis because I still wasn't able to install the BMW TIS software on my laptop correctly.
The Snap On showed the error 'Accumulator Pressure Raises (proceeding upwards out of accumulator)', and after another calibration the struts of the car started to inflate/sag without commands given by the system.

We ended up having the front struts in their highest position, the rears were set to lower than normal. The car's manual levelling leds were on, you could operate them but the car's SLS didn't respond on anything anymore. We took the battery connection off for 40 minutes, then tried calibrating again, as a result the wheels were positioned a little lower than normal but hey I could drive.

Next morning: SLS inactive...

I'm starting to think the valve unit's broken because:

I've read the comments of member esntrk1 regarding the fact that many people overlook the valve unit/pressure accumulator. It's location under the car makes it sensitive for dirt and stones but also for a carwash which cleans the body underneath. A compressor in mint condition, but a valve unit that can't measure the right heights= a good recipe for everlasting faults.

Anyone's having a clue or comment?