21924 is a fault stored in your BMW's memory (an ISTA fault code, not a standard OBD-II P-code).
What it means: Flexray, external cause (group error) — transfer case (LMV, xDrive). This is a secondary/knock-on Flexray-bus entry, not the root cause: the module logged a fault whose source is in another node. On forums it often cleared after transfer-case/gearbox service (fluid change). Usually not serious by itself — fix the root cause if there are symptoms.
