Oh, I had the same problem, same SELFLEVEL SUSP. INACT message and got it fixed finally. My case is kind of weird and I got my air bag (in suspension) and air compressor replaced.

I guess what happened to me was I've once used my X5 to cargo heavy items with trunk lids opened. Based on Selflevel Susp mechanical spec, air compressor may keep pumping while trunk lids opened. So, you may avoid driving with lids opened.

My SA also told me some customers have their SELFLEVEL SUSP. INACT lights up simply because of a failed fuse. There are one fuse in glove box and one in cargo that you can check on.

Hope it helps.