Well, first step would be to verify if this happens as well if you run konqueror http://www.yworks.com/ from a console/terminal. If so, try setting another default browser. (How that is done depends on your system.) In any case, yEd tries to determine the default browser using the which command, testing the following browser names
in the given order. E.g. if which firefox returns a path, the command firefox http://www.yworks.com/ is used instead of konqueror http://www.yworks.com/ to open the yWorks hmepage.