Facebook iFrame App “This form has expired. Please refresh and try again.”

I have build this totally simple Facebook iframe App: https://www.facebook.com/das.boot.wuerzburg/app_518420714894932

The secure URL that is called is: https://www.das-boot.com/facebook

But in facebook nothing is shown - only the following Expression Engine Error is thrown: “This form has expired. Please refresh and try again.”

BUT there isn’t any form or any other ee-tags within the called ee-template only plain html code!?

This error is also new since a few weeks - before everything worked just fine… Could it be caused by an ee-update (I updated a few times from 2.5 to 2.10) or must it have something to do with changes on Facebook side??

Anyone any idea? Going crazy with this…

Thanks in advance