Running Thunderbird 2 with IMAP accounts on a dovecot server, one out of four accounts couldn’t move any files to the Trash. The symptom is a message saying “Could not create folder INBOX.Trash”. Well, right, dovecot doesn’t nest under INBOX.
The workaround is to show the subscriptions for the account and uncheck the subscriptions for the INBOX.Trash and INBOX.Sent folders, then it will properly use the existing Trash and Sent folders.