Calendar Element is dissapeared in editing mode – yet there as outline?

Hello, just learning and trying ForeUI. One of my actual problems: I have two calendars on a page (From / until). Everything worked fine first. Suddenly one calender is invisible (not only when marked visible/invisible) but it cannot be seen in editing mode at all. Only reason i believe, that element still exists in any transcendet form is: it can be found as calendar_3 when i search for calendars. (Find Elements). Even then, outline of element is shown, not the element itself.

The Behavior of the element is: Usually not visible, becomes visible when klicking a text box nearby. (Worked fine and works fine with 2nd Calendar element – but not any longer with the dissapeared first calender).

Maybe i have done some mistakes when introducing some behaviours. But i have no clue where to start, solving the problem.

Thanks for any helps and suggestions.