I'm using centos 6.5, Dovecot, Postfix, and the problem was definitely with my Dovecot imap server. In order to fix the problem I changed the mail location variable in the /etc/dovecot/conf.d/10-mail.conf file to read:
mail_location = mbox:~/mail/:INBOX=/var/mail/%u:INDEX=MEMORY
I've tried this on three different installations of roundcube and it's worked on all of them. Make sure that you restart the dovecot service after making the alterations to the configuration.