app/templates/errors/500.html

{% extends "base.html" %}

{% block content %}
    <h1>{{ _('An unexpected error has occurred') }}</h1>
    <p>{{ _('The administrator has been notified. Sorry for the inconvenience!') }}</p>
    <p><a href="{{ url_for('main.index') }}">{{ _('Back') }}</a></p>
{% endblock %}