These errors are typically generated as a result of configuration or syntax errors in the web server configuration. On Unix hosting site specific configuration is typically placed in .htaccess files which are fundamentally an extension of the server configuration file with parameters specific to your site. They can be placed in multiple parts of your site but their rules will be applicable to content being accessed from within the directory where the .htaccess resides and any directories beneath it until another .htaccess file overrides it.
If your site generates this error following an adjustment to .htaccess it's bound to be the cause of the problem, if you are unsure what the problem is you can opt to disable .htaccess all together by renaming it, then try loading the site again and see if the error persists. Note that .htaccess tends to be there for a reason providing functionalities to your site so removing this should not be seen as a permanent solution but simply part of the troubleshooting process allowoing you to pinpoint the problem. Another step would be to check your error logs, you can enable error logs under web options, make sure to click apply on top of the web options screen then wait 5 minutes and the error log will appear under the /logs dir in your account (Unix Only) Then load your site again and check the error log, .htaccess based problems will typically be visible with a clear message within the log. There is no right or wrong order in which you can use the above steps.
.htaccess syntax itself is an extensive subject and outside the scope of this article.
500 errors can also be the result of errors in the site code itself, they may equally appear in your error log but if you can't find any reason for these errors please contact Quadra Support.
On windows 500 errors can also be the result of errors within code, your web.config file but can be somewhat harder to troubleshoot. If you can't find an immediate cause for the 500 error on windows we can enable detailed errors to show on screen (windows 2008) which will provide a more verbose description of the problem, you will need to contact Quadra support to have this enabled for your site.