Sometimes secure, sometimes not

Hallo Simple SSL Team, greetings from Germany, great Plugin, love it! And for most of my sites it works great. But on this side here, and i really don′t know why, the Plugin does not work? When i first activate the Plugin and then go in the Menu above to “Leistungen” and then on the Drop Down to “Stahl und Metallbau”, the first time Chrome Browser shows the Side is secure. When i go back to the Start side and then return once again to “Leistungen”/”Stahl und Metallbau”, suddenly chrome shows that the Side is unsecure. The same with the other sides in the Dropdown Menu. Do you have an idea why this could be? Thank you very much!

The page I need help with: [log in to see the link]