Hi Fritz,
I see what you mean — that’s no good!
I opened the browser console when looking at your site and it appears there are a number of Javascript conflicts. The tooltips in Month View are created with Javascript, so it’s very likely that some Javascript errors in either your theme or another installed plugin are causing a conflict here.
Before we dive too deep, will you please follow the steps outlined in this post? This will help us locate a conflict if one exists.
Let’s start there and see what we find.
Thanks!
Geoff