Hi Proofy, had the same problem and after a lot of research and testing I found I had a down level version of libsoup (2.48) after upgrading to version 2.50 problem disappeared, so it solved the crash.
Still trying to get my hands around the designer app, not a lot of documentation and/or how-to-use guides.
I am working on OpenSuse 13.2 with KDE desktop for my designer work and found libsoup 2.50 in the GNOME repositories. It no longer crashes, at least not at start, there are some error messages from SOUP and when closing down it crashes in libgobject but that doesn’t interfere with my work in designer, so I live with it.