If anyone's web site has suddenly started reporting "500 internal server error" this may be due to an upgrade at Apache. Check your error log, via control Panel / Metrics, and you may likely find that your server is reporting an error in your .htaccess file. A strong candidate for this error is that you have a comment character (#) that is not at the start of a line. Comments MUST have a # at the start of a line, otherwise they run the risk of being interpreted as arguments for commands (for which # might be a valid argument). This has always been the case, but it seems that Apache has tightened up somewhere, or possibly changed the spec for a particular command, thus causing existing .htaccess scripts to fail.