At roundcube + dovecot, an error message “connection to storage server failed” is returned when users login their mailbox thru roundcube webmail, probably it is caused by limited numbers of connetion per user IP address.

For dovecot imap service on Ubuntu, we can add the following lines to increase to (for an example) 40 connections per user IP address at the end of file /etc/dovecot/conf.d/20-imap.conf if the roundcube is installed at local host.

remote 127.0.0.1 {
     mail_max_userip_connections = 40
}