SSL doesn’t work on certain pages – what is wrong?

If it’s all in one line, it will be commented out at # Redirect HTTP... as the # character is used for commenting in .htacces.

Try:

# REDIRECT HTTP TO HTTPS
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]