apparently the problem is gone. I forgot to remove the jupgrade folder and move all files back to the root of my joomla installation after migrating my site.

But thanks for the quick reply.

For some reason the editor button creates a wrong link (like the one below) which requires users to login before they can download a certain file. If I manually get rid of the /administrator/ then everything works as expected. But I have no clue why the editor button automatically includes the /administrator/ part. Any ideas?

/administrator/index.php?option=com_joomdoc&task=document.download&path=Curricula/Curr_7_8.pdf