Your guestbook may not working if it is trying to call scripts in the wrong location. When the guestbook is loaded into another SWF like Imagevue, it will try to load data/PHP from the root of the SWF file location (which is not the same as the location of the guestbook.swf). I see your guestbook is trying to load this:
http://portfolio.grimur.net/guestbook.xml?uniq=4879
Either this file is not there, or it is protected. Most likely, you have it in your guestbook folder instead. How many files is in this guestbook module altogether? Easiest would be to just move them all to root. Maybe you can just move the XML file to root.
I hope we will be adding a guestbook to Imagevue X2.7. We just have to make sure this is currently feasible, because it means we have to create an entirely new plugin for this, including an admin that can manage all entries.