Thermador’s own documentation describes a “12:00” display as what happens after a power failure or after the unit has been switched off using its own power switch — not an error condition. Re-entering your time and settings after either event clears it.
If you want to avoid this happening repeatedly, Thermador’s guidance is to avoid using the unit’s “Off” option on the clock menu specifically, since that’s one of the documented triggers alongside an actual power interruption.