@Netzhexe, Adding to @Karmalakas, the other site might have twig.autoescape: false in system.yaml, while your current site hasn’t. That could happen if the previous site is upgraded, while the new site is a fresh install.
See the Twig section in Upgrading to 1.7
Use the following in Twig to by-pass auto-escaping for assets using the |raw filter:
{{ assets.css() | raw }}
{{ assets.js() | raw }}
Since auto-escaping is a security measure, I would advice you to go through all your themes and check all output commands in your templates whether the output is safe or not. If safe, add the |raw filter to by-pass auto-escaping.
When done and well tested, I would un-set twig.autoescape in system.yaml for sites that have auto-escape switched off.
Have a look at Quark and see which outputs use filter |raw and which don’t.