Well, I just have downloaded both the tutorial and GUI manual from SourceForge and everything worked well. I assume there are some problems with some of the mirrors (we got several messages about this problem) and therefore we decided to additionally host the documents directly on our web server. Please check our download page at http://rapid-i.com/content/view/26/82/
and you will now find two mirrors for both documents.
Thanks to all people pointing this problem out.