That path seems correct, the error is produced is just a side effect of printing something to the screen before the session_start call, so no need to focus on that.
The issue seems to be your web server is changing the directory that PHP thinks it is in, so all of the require/include/file_get_contents lines that use relative paths are incorrect. My guess is the fix would have to be adding dirname($_SERVER["SCRIPT_FILENAME"]) . to any path throughout the files.
That will likely take a while though, so a better way would be to ask your web host why the directory is being changed on the web server. I am assuming they have a setting in place that is causing this issue.