At a guess, your dealer is at fault, it sounds like they did not do the correct power down procedure on you nav unit.

They must have disconnected the battery while the unit was writing to flash, this will have corrupted the flash memory on the unit, its now toast.

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Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.