Well..... it seems I draw to a conclusion too soon... it's not just my template. Other templates have this problem too.
Yesterday it only seemed that my template in a clean install did have this problem. Now I don't know what my problem is.
After some investigation with a clean install I don't know what the problem was. I copied my template and content.htm back to the clean install and everything works fine. I don't know where the problem was.... some strange problem in my old CMSimple installation, which has been patched for some years now.
I'm gonna go for the new installation now. With my old template and content. Thanks for the support!!!

