Problem with Redirection to WWW

Backlight 4, 5 & 6 Support
#1

I have for my site a redirection from non-WWW to WWW in my .htaccess:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^designladen.com [NC]
RewriteRule (.*) https://www.designladen.com/$1 [R=301,L]

This works fine for alle pages which are not using backlight
For my backlight pages the redirection does not work:
https://designladen.com/fotos/
https://www.designladen.com/fotos/

Thanks in advance for your help!
Zippo