Best guess is there’s something configured very differently between the two site in the .htaccess and or php.ini file. Honestly, I can’t even begin to guess what the issue may be.
Are you sure you’re using the same PHP version between the two sites? I know I can configure which version on a per site basis.
Any reason you’re using the ancient version of PHP? The oldest I can even test with and the version I do the dev work on is 5.2.17. I can’t see how that would make a difference… but it is PHP so it could be the issue.
Have you checked the PHP error logs? They may reveal more details that could be helpful?