There are, of course, lots of causes for a 500 error. Most often, this error originitates from a poor configuration of a .htaccess file i.e configurations in the file are not compatible with settings on the server on which your Drupal website has been installed. php.ini, htpasswd files that are wrecked are also possible causes of this famous problem. However, breathe! Most of the time, this problem can be fixed by doing a little bit of work in your .htaccess file
Always start by checking out your apache/nginx log files as they are the best tools to pin-point the root cause
If you are using PHP5 then in your .htaccess file, locate the following block
# PHP 5, Apache 1 and 2.
php_value magic_quotes_gpc 0
php_value register_globals 0
php_value session.auto_start 0
php_value mbstring.http_input pass
php_value mbstring.http_output pass
php_value mbstring.encoding_translation 0
Ensure that the settings match the server settings or betting still, comment them out or eliminate them completely and try to load the page again.
If that does not do the trick, then try to check whether PHP safe_mode is set to ‘on’
The PHP safe mode is used to solve the shared-server security problem. Go to /drupal_server/Uniform Server/udrive/usr/local/php.ini and open it with a text editor. Look for the following code: safe_mode=On. Alter the mode to off by replacing “On” with “Off.”