Problem

I recently upgraded from 13.04 to 13.10, and suddenly notification bubbles don't appear themed as usual in the upper right corner, but they appear as white text on blue background in the upper-left corner. It looks like this:

Unsuccesful attempts to fix it

• I tried reinstalling unity, notify-osd, ubuntu-desktop removed notification-daemon which was installed, none of that fixes it.

• In fact running ps aux | grep notify-osd shows that notify-osd isn't even running. But when I try to start it manually by running /usr/lib/x86_64-linux-gnu/notify-osd I get:

  ** (notify-osd:4618): WARNING **: Another instance has already registered org.freedesktop.Notifications
  
  ** (notify-osd:4618): WARNING **: Could not register instance
  

• If I understand well, the instance is registered by the /usr/share/dbus-1/services/org.freedesktop.Notifications.service file, which right now contains:

  [D-BUS Service]
  Name=org.freedesktop.Notifications
  Exec=/usr/lib/x86_64-linux-gnu/notify-osd
  

  Renaming or deleting that file (and rebooting) has no effect whatsoever (and it is not recreated automatically).

• This is not a duplicate of No notifications from notify-osd on 13.10 (and by the way I purged gnome-flashback-session along with notification-daemon)

Question(s)

How can I debug this? How can I get notifications to come back to normal?

If additional debug information is needed, I'll be happy to add it (just that I can't find any more).