If server supports namespace then you just should set these options to null. Remember to do a re-login in Roundcube after change.
Enable imap_debug in Roundcube and send the log here (after you change these settings and log out).
these part of the log is maybe important as well...
[09-May-2012 12:12:08 +0200]: [69BC] C: A0002 NAMESPACE [09-May-2012 12:12:08 +0200]: [69BC] S: * NAMESPACE (("INBOX." ".")) NIL (("#shared." ".")("shared." "."))