this is a common and known problem
the problem is either a fault in the CAS (car access system) or with the steering lock itself.
sometimes, a software update and new battery (along with charging system test to verify voltage never gets too low) can resolve the issue, however, more commonly, the steering column needs to be replaced (the steering lock is integrated into the column)
it is somewhat shocking that your dealer cannot suggest a path forward even without the diagnostic machine showing faults - your description is hallmark for a known issue, and the known fixes are as I've described above. I would consider finding a new dealer or complaining to BMW corporate customer service.
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