Although Exception: Websites don't load properly

Hi there!

We have specific websites that we need for our teachers to work with but there is some weird behavior when using the webfilter. Without filtering in the network it is working great.

I.e. Interactive ebooks for children

Open the URL. With webfilter turned on it takes ages to load. And When clicking the green button "teacher login" on the upper right it just adds a hash character behind the URL and nothing further.
Without the webfilter a login animation/window appears on the site. That is how it should be.

I put raz-kids.com on the exception list (this can be proofen by the filter log) but somehow the webfilter does something to those pages...

First we thought it's because of some javascript... but even the option to remove Javascript content has never turned on on our Sophos UTM425, v.9.103-5

Please help!! :-/
--Uwe