The first time I had the message 'SELFLEVEL SUSP. INACT' showed up on my 2005 X5, I took it to the dealer and it was fixed under warranty. The car was tilting to the right when the message showed up. If you look at the car from the front, you will notice that the car is tilting to one side. 4 years on, the problem reoccured but this time the car was tilitng to the left and the message 'SELFLEVEL SUSP. INACT'. With labor, it is costing me around USD 1,200 to get this fixed and they will be replacing a part. I will know more when I receive the car in a few days and I will post the exact response then. In my case, the problem occurs along with the tilting which is VERY noticable.
Looks like it is time to sell the car since this is the first time I ever paid anything for my X5 after 4 years of owning it (warranty 3 years and free service for 5 years or 100k). Due to credit crunch, cars becoming less expensive where I live (Dubai).