The %20 here suggests that there is a space being inserted into the name that probably doesn't belong there. (I'm assuming the location /bootstrap/ckeditor/plugins/mathjax without the space is the correct location on your server). I'm wondering if you added a space after the comma in 'youtube,mathjax'?
In any case, this is a creditor issue, not a MathJax one, so you should ask that community. I don't use creditor, and just looked at the documentation since your javascript didn't look right to me. We will not be able to help you further here.
Davide