Sorry for the late reply.
I think the problem is just the menu URL generated by Joomla. If I click on the "Alle Veranstaltungen" ("All Events"

button in the frontend module or if I place the menuitem inside a visible menu, everything is fine. I noticed a difference in the url:
index.php?option=com_jevents&Itemid=331&task=list.events (button)
index.php?option=com_jevents&view=list&layout=events (link provided by menu in backend (hidden menu))