Thank you @jonathan and @MarkHunte for helping me out with this. Tried your suggestion @MarkHunte and it works perfectly when running the project using a web server. As it is running locally and I have to figure out whether opening a web server is possible, i chose @MaxZieb s advice from Remove Event Listen on Scene Load - Using JavaScript with Hype - Tumult Forums the onmouseup-event meanwhile.
Thank you guys again, you're awesome!