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Well, look at the difference between the two (besides the IfModule). Before you had RewriteRule ^(.*)$ profile.php?$1 [L,QSA] That would turn "johndoe" into "profile.php?johndoe". That's not what you wanted. Now you have RewriteRule (.*) /profile.php?id=$1 [L] That will produce "profile.php?id=johndoe". Keep in mind this will kick in for any path that doesn't exist - not just "words". You should consider limiting exactly what this can match.

PHP is the cart, your web server is the horse. You cannot put the cart before the horse. Pick the simplest .htaccess you've tried and post it here, then we'll see what's wrong with it. Can I also assume that you, at some point during these 500 errors, looked in your server's error logs for an error message? What did it say?