Aside from the yaw sensor going bad I seem to recall seeing some posts about the steering angle sensor (SAS) needing to be re-calibrated. After battery has been disconnected or drained the X no longer knows the position of the steering wheel as read by the SAS. Without it the DSC system is locked out. There are two levels of SAS re-calibration: one you do simply by turning the steering wheel full turns lock to lock but another via GT1 only where you point it straight ahead and reprogram it. I also recall in GT1 you can see output from SAS to verify it is working. You can also see output of yaw and wheel sensors that can be used to verify them too.

best of luck

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2003 E39 530i
1992 MX5
2005 E53 x5 4.8is (sold)
1990 K75S (sold)