Do your logs give you any clues?
/usr/local/cwpsrv/var/services/roundcube/logs/
especially /usr/local/cwpsrv/var/services/roundcube/logs/errors.log
grep storage /usr/local/cwpsrv/var/services/roundcube/logs/errors.log
Usually storage server errors with Roundcube indicate an IMAP connection issue. Is dovecot functioning normally? So also check out
/var/log/dovecot-debug.log
/var/log/dovecot-info.log
/var/log/dovecot.log