Hi,
Can you enable the debug mode?
With debug mode enabled, a detailed error message will be shown instead of the error 500, it will be easier for us to find the problem if you provide us the error message shown when you try to edit.
To enable debug mode you have to edit 'conig/config.php', at the definition of the DEBUG constant, change its value to true.
define('DEBUG', true);
please tell us when you get the error detail.
regards