nevermind, fix found (shown below just in case anyone else might find it useful):
Just found the file in the Joomla 1.5 package. It’s the index.php in the root directory. Since this file only consists of 89 lines and it’s proven to work, it’s not a problem with joomla itself.
Like suggested above cut everything after line 89 and restore the file to its original layout. If unsure just load the latest Joomla 1.5 installation and take the included “index.php” file. Btw. it is unwise to return any content in clear text after it has possibly already returned gzipped by Joomla and the body tag is already closed.