Comment # 6 on bug 1158543 from Fabian Vogt

(In reply to Dr. Werner Fink from comment #5)
> One problem seems to remain: libdbus1 aborts sddm if an second X server
> (which crahses) started over display-manager service.

Why do you think that's libdbus1?

sddm has:

>         // start display server
>        if (!m_displayServer->start()) {
>            qFatal("Display server failed to start. Exiting");
>        }

By default, qFatal calls abort() after logging. Feel free to open an issue
upstream (https://github.com/sddm/sddm).