500 Internal Server Error

I have tested my site by downloading the PHP file from this site, and my site was working perfectly with the Jooma SEO option. 

After installing SEF Advance, I get an error when I click on any of my menu links (Please advise):

500 Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, webmaster@localhost and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

And yes, I have read the FAQ and have checked that the htaccess file was uploaded properly.