As you might have heard, also the developers of Roundcube are just human beings and thus not always creating perfect and 100% bug free work. The last 0.4.1 release fixed many bugs but also introduced a quite annoying new one. That's why the next release follows just one week later. It fixes a few important bugs, especially the "Server Error" message when expanding/collapsing folders and it also updates some previously incomplete localization files.
More... (http://sourceforge.net/news/?group_id=139281&id=292531)