The release upgrader was unable to calculate the upgrade path because the package thunar-data which you appear to have installed from a PPA.
2013-05-21 09:43:22,169 DEBUG Foreign: xfce4-settings thunar-data libexo-helpers libexo-common libexo-1-0 virtualbox-4.1 exo-utils google-chrome-stable parole mousepad xfce4-terminal
and
2013-05-21 09:43:22,262 DEBUG examining: 'deb http://ppa.launchpad.net/xubuntu-dev/xfce-4.12/ubuntu precise main' 2013-05-21 09:43:22,264 DEBUG entry '# deb http://ppa.launchpad.net/xubuntu-dev/xfce-4.12/ubuntu quantal main # disabled on upgrade to quantal' was disabled (unknown mirror) 2013-05-21 09:43:22,264 DEBUG examining: 'deb-src http://ppa.launchpad.net/xubuntu-dev/xfce-4.12/ubuntu precise main' 2013-05-21 09:43:22,267 DEBUG entry '# deb-src http://ppa.launchpad.net/xubuntu-dev/xfce-4.12/ubuntu quantal main # disabled on upgrade to quantal' was disabled (unknown mirror)
The release upgrader was unable to calculate the upgrade path because the package thunar-data which you appear to have installed from a PPA.
2013-05-21 09:43:22,169 DEBUG Foreign: xfce4-settings thunar-data libexo-helpers libexo-common libexo-1-0 virtualbox-4.1 exo-utils google- chrome- stable parole mousepad xfce4-terminal
and
2013-05-21 09:43:22,262 DEBUG examining: 'deb http:// ppa.launchpad. net/xubuntu- dev/xfce- 4.12/ubuntu precise main' ppa.launchpad. net/xubuntu- dev/xfce- 4.12/ubuntu quantal main # disabled on upgrade to quantal' was disabled (unknown mirror) ppa.launchpad. net/xubuntu- dev/xfce- 4.12/ubuntu precise main' ppa.launchpad. net/xubuntu- dev/xfce- 4.12/ubuntu quantal main # disabled on upgrade to quantal' was disabled (unknown mirror)
2013-05-21 09:43:22,264 DEBUG entry '# deb http://
2013-05-21 09:43:22,264 DEBUG examining: 'deb-src http://
2013-05-21 09:43:22,267 DEBUG entry '# deb-src http://