secondary menu mysteriously disappears.

Hey everybody,

I have strange things going on: On one of my pages the secondary menu in the left sidebars disappears, I have no explanation for that behavior as it does not repeat on other pages.

Could somebody have a look at this?

https://austrocult.si/veranstaltungen-im-mai2/ (works)
https://austrocult.si/aktuelle-themen/ (works)
https://austrocult.si/andere_veranstaltungen/ (menu disappears)

Thanks for your help,

rich