I read through this thread... and found what I think may help you.
In a shared web host setting dependent upon how/where the source was built/retrieved from it is most likely a permissions problem in one of the folders in the 'chat sub-directory. So far I have run across this in several places because the permissions were not '755' . I am working to create my own branch and these perms have proven difficult for me to get correct when I create a zipped archive from the sources in my local git repo which is a remote of
https://github.com/MotoHoss/AJAX-Chat,
I ran into this using my very alpha branch i had to go through all dirs and fix perms by hand. Maybe as I get familiar with what they are supposed to be exactly... and how to process them properly when I create a bundle it will help. I ran into this on the drupal branch as well. it uses 0.8.7 (IIRC). I would check the perms on a working chat... no matter the branch.. and make the perms the same in the one that isn't working... and you should be good to go. (i.e all the sub-dirs).
Edit(removed prior posts) oops.