Hi,
I don't own a X5 but have had similar problems on my RR.
First if you install a back-up camera under the rear spoiler, it will be close to the GPS antenna and if the camera wires are live, it will disturb the GPS antenna and give a reciever error.
My problem was a broken antenna extension cable; when connecting The GPS antenna directly into the MKIV all worked fine.
I hope this information could be of some use.
Regards