If you view the source of your page, you are sending a second <title> </title> tag - which makes the HTML on the page invalid. Threads sends it's own Title tag so if you have this code in your header, remove it.
In fact, view your source, your pages have 2 <head> tags.
The "Default Header" should only be HTML. NO meta tags or <head> tags etc....
The "Header Insert" section will appear inside of the <head> tags. This is for javascript or meta tags. There shouldn't be any html output in there. Don't use the <title> tag in there because threads has it's own title tag.
That's why yours is broeken.
