How about validation? If I trie it with http://validator.w3.org/ there are a lot of errors like this: "Element div not allowed as child of element h3 in this context. (Suppressing further errors from this subtree.)".
http://wordpress.org/extend/plugins/all-in-one-event-calendar/
Thank you for reporting this Azubi.
We are quite in early stage with this plugin (still a 1.X release) but passing w3.org validator is a good thing to have. I will add it to the list and hopefully we will clean the errors soon.
With new version 1.1.2. there is still the same trouble with the validation.
We improved it a bit but we are working on a new monthly view design that will pass the validation.