If you see error "500 - Internal server error" generated by your webserver after you have installed the AceSEF component or after you did some adjustments in your .htaccess file, there is probably some misspelling or invalid configuration in the .htaccess file, or the .htaccess file usage is not allowed in your environment.
Most often, the cause is usage of some unsupported directive. Some servers may generate this, when Options +FollowSymLinks line is enabled (uncommented). This will occur when redefinition of Options is not allowed within .htaccess file because of security reasons. In such case, you will need to keep the line commented (in most cases you anyway do not need this activated).
Another reason for 500 - Internal server error may occur, if you have updated the .htaccess file. You may have added unsupported directive or made a spelling error when editing it. Double check all your adjustments or try to revert them.
In some environments, usage of .htaccess file may be prohibited globally by server administrator. In such case, just creating the file (AceSEF attempts to create the file automatically during installation) may lead to 500 error. If this is your case, rename or delete the .htaccess file to get your site running again and consult your server admin about the next possible steps.