Roundcube Mailbox doesn't exist: Trash
Have a look in /var/lib/roundcube/config/defaults.inc.php (in Ubuntu 16.04.1) if the following lines exist and are correct
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// store draft message is this mailbox
// leave blank if draft messages should not be stored
// NOTE: Use folder names with namespace prefix (INBOX. on Courier-IMAP)
$config[