Migrated website empty

Website URL

QuizWalk.gamer.gd

Error Message

185.27.134.xxx returned an empty site

Error code: 500 Internal Server Error

Other Information

Hi. I migrated a working site to InfinityFree four days ago and the domain name is active in my DNS – I can get the file index2.html (index.html) to display, but not my own index.php. I uploaded all the PHP and Java files, etc., got a new Captcha for the InfinityFree domain name and updated the keys, plus uploaded a MySQL database, which looks fine on phpMyAdmin. Problem is, I get a 500 Internal Server Error with the domain name when I use my index.php file. When I enter the IP4 address in the browser, it says the host sent back an empty page. Index.php is in the folder htdocs, unchanged except for Captcha, and is maybe 4-5 years old (PHP/Java/JS versions unknown). What could the problem be? Thanks!

P. S. I edited the index.php file so it had one echo command and the rest was a comment, and it worked. So what could be the problem with the index.php?