Hi,
Take the htaccess.txt at the root of your website, or in a brand new Joomla installation package and replace your current .htaccess with it.
That should fix the 500 error if it comes from that.
Otherwise, it means that there is a PHP error and you should look in your PHP error log of your web server for the error message relative to the crash of the processing. That will give valuable information on what's going on.
Security components can potentially detect false positives (thinking that a request is an attack because it detected something in the request which is actually not a security problem). To avoid these only two solutions: either deactivate the security check which causes the problem and modify the code of the security component to not trigger that false positive detection.