I have the following code in an htaccess file in my application root to turn output buffering on.
php_value output_buffering On php_value output_handler mb_o
You can use this syntax only if PHP is running as an Apache module.
The 500 errors probably come up on servers where this is not the case.
For total certainty, look into the server's error.log file for a detailed error message.
error.log