I had this problem with my E39, and have it now with my X5. It is apparently a very common problem.

The solution on my E39 was to replace the original hose clamp (going to the bottom of the Power Steering fluid reservoir) with a better one. I used the worm gear style clamp available at any parts store. I haven't taken the time to do the X5 yet, but it has held for 6 years now on my E39. Maybe try that first before replacing any hoses.

I found it easier to simply open the OEM clamp up, and move it further down the hose, rather than try to remove the clamp entirely.