The "gdk_mir_display_open" error is just GDK/GTK trying to use Mir because it found no X11 server ($DISPLAY is blank). That's not a Mir bug.
You are not trying to use Mir, and Mir does not seem to be responsible for the breakage either. If the system was not broken then $DISPLAY would be set to something correct (like ":0") and GDK would not be trying to fall back to Mir.
The "gdk_mir_ display_ open" error is just GDK/GTK trying to use Mir because it found no X11 server ($DISPLAY is blank). That's not a Mir bug.
You are not trying to use Mir, and Mir does not seem to be responsible for the breakage either. If the system was not broken then $DISPLAY would be set to something correct (like ":0") and GDK would not be trying to fall back to Mir.