Delete the folder <documentroot>/app/cache/prod and <documentroot>/app/cache/dev if they exist. Normally such a 500 error is an attempt to load a class that is already loaded in the cache.
If that does not help, have a look at your php logfiles. On unixboxes, normally look in /var/log/php* (* for the version).
Otherwise put error reporting on (can be nasty in the front end too on production systems...). Edit <document root>/config/defines.inc.php and change the second line:
if (!defined('_PS_MODE_DEV_')) {
define('_PS_MODE_DEV_', true);
}
This will give you a cause why the error occurs.
By the way, when deleting modules manual in the database, do no forget the named entries in ps_authorization_role table. The SLUG is something like ROLE_MOD_TAB_<MODULE NAME>_<ACTION>.
Succes,
Leo