It’s a bug, execute these commands as root and try to access webmail again:
chown -R hestiamail:www-data /etc/roundcube/
find /etc/roundcube/ -type f -iname "*php" -exec chmod 640 {} \;
chown -R hestiamail:www-data /var/lib/roundcube/
chown -R hestiamail:www-data /var/log/roundcube/
If you will use phpmyadmin, execute these commands too:
chown -R root:www-data /etc/phpmyadmin/
chown -R hestiamail:www-data /usr/share/phpmyadmin/tmp/