Comment # 5 on bug 1000917 from Fabian Vogt

The crashing code snippet:

// determine session type
const QString &autologinSession = mainConfig.Autologin.Session.get();
Session session;
if (findSessionEntry(mainConfig.XDisplay.SessionDir.get(), autologinSession)) {
    session.setTo(Session::X11Session, autologinSession);
} else if (findSessionEntry(mainConfig.WaylandDisplay.SessionDir.get(),
autologinSession)) {
    session.setTo(Session::WaylandSession, autologinSession);
} else {
    qCritical() << "Unable to find autologin session entry" <<
autologinSession;
    emit loginFailed(m_socket);
    return;
}

which means the session entry defined in your sddm configuration cannot be
found.
Can you upload your sddm config? Does it work again after you disabled
autologin?
The crash itself is only a symptom (which can be easily fixed), but it went
wrong before.