Okay so the non-responsive css file wasn't exactly what we were looking for. However, we found the problem:
IE8 doesn't seem to like the @import-property.
We used a global css file which imported the necessary styles. When moving all stylesheets to the header and including them with a normal link-tag, it works.
It's weird though, the regular bootstrap styling (buttons, backgrounds and such) got imported properly. The grid system was not. I guess Internet Explorer works in mysterious and retarded ways.