I ran into this issue recently where users were getting the error:
“Unexpected condition from IMAP server, closed or corrupt connection to IMAP. Possible mailbox corruption.”
First, try a:

 mysqlcheck --auto-repair

This will more than likely yield “The storage engine for the table doesn’t support repair” Due to it’s engine used, this is fine. What you may be looking for is “error : Table upgrade required. Please do “REPAIR TABLE `session`” or dump/reload to fix it!”
It may occur on others as well.
So now, we’re going to make raw-data backups

cp -pr /var/lib/mysql/roundcube /usr/src/roundcube_backup
mysqldump roundcube > roundcube.sql
mysql -u root roundcube < roundcube.sql

The above should fix your problem, or point you to an easily fixable corruption in the sql file.

else, lets log in to WHM and select mailserver configuration:
Main >> Service Configuration >> Mailserver Selection
and select dovecot here. It will take a second to build, and this may also potentially help.

Enjoy.