I've actually found the real solution for this error quiet a while ago but forgot to post it here...

The JS Interpreter of ServiceNow cannot process JS String template literals. I went through the ChartJS Code and replaced all template literals with with concatenated Strings manually. After that it worked fine as a UI-Script. 

I've even made a little script which replaces the template literals automatically. If you are interested you can get it here:
https://github.com/gr3enk/NowChartJS