What could be the cause of interrupting the drive during start-up if the internal bell rings and there is no indication of closed doors?

The use of the emergency door release mechanism by a passenger causes the simultaneous activation of an audible signal (a bell) and a lock which prevents the vehicle from moving off. This is a standard safety system in public transport vehicles that prevents driving with the doors open, which explains both the alarm and the interruption of the vehicle's start-up.