The problem with the HTTP Status Code 500 are very difficult to fix errors, because the 500 error summarizes many potential errors.
The most common cause is a faulty. Htaccess. These files can be cleaned with a Debug log file.
Another reason may be a PHP error. There are certain reserved identifiers, which must not be used as variable names as: list. There is also a 500 Internal Server Error thrown.
$list($a, $b) =functionA(); //richtig ist: list($a, $b) =functionA();