Hey again Jesse, thank you so much for putting an effort into helping me, I really appreciate it!
I have the "Selectable parent categories" checked since I installed it, haven't even unchecked to check it's function. Didn't wan't to mess with stuff I don't get, at least not until I have a stable version to backup
This also occurs in IE, but not with the categories, but with the locations. I'd love to understand more of the Osclass structure, but I'm having some trouble fixing the hard coding parts of this system, I'm more into designing and css-coding, but I enjoy learning some more php and mysql.