Design-horror on some pages, not sure whether it comes from the said plugin

What you see on the screenshot is that there is no more menu in the header. The text you see written all over the top of the page comes from one of the "event cards" ("Mephisto":wink:. This happens here:

As it does not happen here where the same calendar plugin is in place I am not sure whether or not it is caused by THIS plugin. (eventON)