I was having "(...) unable to handle this request. http error 500" and found out it was from a require_once that was working locally, on a windows machine, with backslash (\) as separator for directories but when i uploaded to my server it stopped working. I changed it to forward slash (/) and now is ok.
require_once ( 'cards\cards.php' ); // **http error 500**
require_once ( 'cards/cards.php' ); // OK
