It sounds like Kubuntu is trying to open it's default homepage URL in a browser session when KDE starts, but the path to that file is no longer what it was originally expecting. In other words, your directory structure has changed, e.g. perhaps you installed Kubuntu to a hard drive? In any case, it may be launched via a script in your /home/$YOUR_USER_NAME/.kde/Autostart folder. Is there anything inside that Autostart folder?
-AT