The "500 internal server" error message just means: "Your web server just crashed". There are many different things that can have gone wrong.
To determine the cause, you need examine your web server logs (your hosting provider can tell you where these are if you don't know where to find them).
Below is a list of some of the most frequent causes of a "500" error on a Drupal web site (I am trying to make this generic - for future reference - I know you've covered the PHP memory limit already):
The setting for PHP memory_limit is too low. Note that if you're on shared hosting, you can't change this yourself. You need to get it increased by your hosting provider.
Your hosting provider has set restrictions on what php_values or php_flags you can put in .htaccess that is violated by one of the .htaccess-files created by Drupal or by directives added to these by you.
Your have syntax errors in one of the .htaccess-files in your configuration. (In particular if you've edited the default .htaccess.)
There exists permissions on files or folders that is outlawed by a security measure your hosting provider uses.
As the message says: "The server encountered an internal error or misconfiguration". To gather information about what type of error this was, you need to examine the server (error) logs. There is no way we can help you with this without seeing the relevant error messages from your logs. We don't have access to your server or your logs.
You may also increase the level of error reporting from PHP to get better diagnostics. You'll get full runtime error reporting by adding this to the start of settings.php on your site (just below <?php):
error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);
In addition, navigate to: Administration → Configuration → Development → Logging and errors and select the radio button "All messages". This sets $conf['error_level'] = 2;.
When you examine the logs and diagnostics from PHP error reporting, you may find out more about what made your server crash. If you do not fully understand what those messages means, you can update the question with the specific error messages, and hope that someone here shall be able to shed more light on the cause of the problem.
If everything else fails, make a backup of your database (you may use the Backup and Migrate module or some DB tool that works independent of Drupal (e.g. phpMyAdmin). Then disable and uninstall the module that you thinks is the cause of your problems (sound like it is the Advanced Forum in your case. Make sure you not only disable, but also uninstall (these are two separate steps).
Verify that your site is now stable.
If it is stable, re-install the module (e.g. Advanced Forum) that caused the problem from scratch.
Re-assess the status of your site. Is it still stable? Is any content missing?
If it is not stable, then the problem is not solved, and you need to reconsider the use of module that causes grief (i.e. ditch it and find a better module).
However, if content is missing, you should be able to restore that back from the back-up yiu made (perhaps after looking through your backup files for anything problematic, and deleting it from the backup before restoring).
