first of all what does ~/post_logon.sh script do? can you post it's contents on pastebin and then post the link here? also we really need to see the Xorg.0.log file to know what the error is. and maybe even ~/.xession-errors file as well. if you only have access to tty1 than you may be able to install pastebinit with
then issue the following to upload the log file using
sudo apt-get install pastebinit
then write down the links it spits back at you and post those links here.