kmail: Unable to fetch item from backend

Error in kmail:

Further symptoms:
- Selecting an email causing the above error does nothing in ways that can be very misleading to the end user. The error message above is hidden in the status bar at the very bottom of the window and can easily be missed. On the other hand, the message window still shows the last successfully loaded message. Depending on the content of the message, the user can be tricked into thinking that the message she is seeing is the one she actually selected.
- No clear, obvious error message is displayed.
- The message is (logically) not set as read.
- Trying to delete the message does nothing. The message stays there in the message list forever.

The only known workaround, is to open a console and browse to the actual Maildir directory containing the message, and deal with it by hand (e.g. using vim to read it, mv to move it or rm to delete it). This can be a difficult proposition in a folder containing a large amount of emails: one has to identify the email's actual file name as used on the filesystem.

In a typical mailbox folder, there are three directories:

Normally, new/ and tmp/ should be empty. When the above error occurs, it is observed that new/ is not empty.