I have on of my installs on GoDaddy. They're really screwy as far as this script goes, not sure why.
It worked fine until one night it decided to stop...The evidence has all but danced naked in front of their faces that they did something to screw it up, but they still say it's something wrong with aC (even though it worked fine at first and an exact copy works fine on a different host...grr)
My suggestion - find a new host for your aC installation.
I would start by re-uploading activeCollab files and see if that helps. It may be that one of the files is partially uploaded and makes other files opening and closing tags incomplete or something like that.
you can see it:
http://labs.nsslive.net/activecollab/
PHP is installed correctly, i have a ton of other apps running fine.
it's shared hosting on godaddy.
any ideas?