500 internal server error

As I said SEF Advance has nothing to do with this! Check your .htaccess and mod_rewrite settings.

Something must have changed by your host! SEF Advance can not possibly stop, it's not controlled by outside. Check your .htaccess and remove it to see that's the problem.

never changed anything in my htaccess files and my server settings remain the same

This is the message that is displayed on the Internal error 500 page

Apache/2.2.13 (Unix) mod_ssl/2.2.13 OpenSSL/0.9.7a mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 Server at www.mysite.com Port 80

and here is a text dump from my htaccess

##
# @version $Id: htaccess.txt 9975 2008-01-30 17:02:11Z ircmaxell $
# @package Joomla
# @copyright Copyright (C) 2005 - 2008 Open Source Matters. All rights reserved.
# @license www.gnu.org/copyleft/gpl.html GNU/GPL
# Joomla! is Free Software
##


#####################################################
#  READ THIS COMPLETELY IF YOU CHOOSE TO USE THIS FILE
#
# The line just below this section: 'Options +FollowSymLinks' may cause problems
# with some server configurations.  It is required for use of mod_rewrite, but may already
# be set by your server administrator in a way that dissallows changing it in
# your .htaccess file.  If using it causes your server to error out, comment it out (add # to
# beginning of line), reload your site in your browser and test your sef url's.  If they work,
# it has been set by your server administrator and you do not need it set here.
#
#####################################################

##  Can be commented out if causes errors, see notes above.
Options +FollowSymLinks

#
#  mod_rewrite in use

RewriteEngine On


#  Uncomment following line if your webserver's URL
#  is not directly related to physical file paths.
#  Update Your Joomla! Directory (just / for root)

# RewriteBase /


########## Begin - Joomla! core SEF Section
#
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !^/index.php
RewriteCond %{REQUEST_URI} (/|\.php|\.html|\.htm|\.feed|\.pdf|\.raw|/[^.]*)$  [NC]
RewriteRule (.*) index.php
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization},L]
#
########## End - Joomla! core SEF Section


########## Begin - Rewrite rules to block out some common exploits
## If you experience problems on your site block out the operations listed below
## This attempts to block the most common type of exploit `attempts` to Joomla!
#
# Block out any script trying to set a mosConfig value through the URL
RewriteCond %{QUERY_STRING} mosConfig_[a-zA-Z_]{1,21}(=|\%3D) [OR]
# Block out any script trying to base64_encode crap to send via URL
RewriteCond %{QUERY_STRING} base64_encode.*\(.*\) [OR]
# Block out any script that includes a <script> tag in URL
RewriteCond %{QUERY_STRING} (\<|%3C).*script.*(\>|%3E) [NC,OR]
# Block out any script trying to set a PHP GLOBALS variable via URL
RewriteCond %{QUERY_STRING} GLOBALS(=|\[|\%[0-9A-Z]{0,2}) [OR]
# Block out any script trying to modify a _REQUEST variable via URL
RewriteCond %{QUERY_STRING} _REQUEST(=|\[|\%[0-9A-Z]{0,2})
# Send all blocked request to homepage with 403 Forbidden error!
RewriteRule ^(.*)$ index.php [F,L]
#
########## End - Rewrite rules to block out some common exploits

AuthName "Secure Area"
AuthType Basic
AuthUserFile /public_html/downloads/.htpasswd
require valid-user



This for me is a major major issue please help!! ???

Thanks for your help so far. I know you're adamant that this is not an issue with SEF Advance. But my host thinks otherwise and I'm caught in the middle. It seems strange that this has happened for three people today. Soemthing has gone wrong. Any further ideas you have for sorting this out would be much appreciated. My host came back to me with the following:

"As far as we are aware, our testing shows this to be due to an incompatibility between SEF Advance and the PHP Zend Optimizer version 3.3.9.  As such, this is not an issue with .htaccess files.  We recommend that you remove SEF Advance at this time until a fix for this issue has been released. "

Maybe they are on the same host, I don't know. SEF Advance can not possibly cause this.
They say incompatibility with Zend Optimizer. How did it work yesterday then? In that case they added Zend Optimizer or upgraded it. In that case it would be a confict with ionCube, not SEF Advance.

Still nobody replies what happens when you remove or rename .htaccess ???

I removed htaccess and get the same error. Thanks.