Further to what @gate2vn said, also check the logs in /var/log/ , in particular the mail.log files and the dovecot log files. Fact you're getting the LIST and SELECT errors suggests it's an IMAP problem, whereby roundcube can't access the mailbox. Things to check are the logs, the permissions of the /home/<user>/Maildir folders and also if the folder for the mailbox you are trying to access exists. Mailbox could also be corrupt. Check the logs first for a better idea and one of us could probably help you further.