Page Title - Warning missing argument 2

Hi there,

I just encountered a weird issue after I changed themes. Just don't know for sure if that caused it, but I have now on some of my pages a missing argument error instead of a normal page title.

It started with a random WP page, which I resolved after deleting and remaking it. But now I have it again.

Checkout the screenshot to see what I mean. Or visit to see it for your self.