ERROR: Could not complete request. Query: SELECT “INBOX.Sent” Reason Given: Unable to open this mailbox.

If you getting the above error while clicking on the sent items in the Webmail (Horde or SquirrelMail ) then it means that the tmp folder is missing under the .Sent folder, So just create the tmp folder under the .Sent folder and assign the ownership of cpaneluser.cpaneluser to tmp folder and just refresh the browser.

That’s all you are done.