Comment # 5 on bug 1165646 from Stefan Hundhammer

With the help of @snwint we found out that at least during installation, the
YaST start scripts at some point call /usr/lib/YaST2/bin/testX which also tries
to start the "icewm" window manager.

We don't know yet if this is also what the start scripts starting
yast-firstboot do (but I guess they do).

When I run this testX in a running Xfce4 environment, I get an error

  testX: could not run windowmanager
  fatal IO error 2 (No such file or directory)
  on X server ":0.0"