When using the dialog to configure KMail and specific the Local Folders, using
Change to edit the configuration of Local Folders, the text field with the name
of the maildir folder shows /home/freek/.local/share/akonadi_maildir_resource_0
. This maildir folder contains a number of messages in subfolders .../*/cur .
However these massages are not displayed in KMail. KMail shows other messages
in trash. Searching in ~/.local/share/ I found a folder 
~/.local/share/.akonadi_maildir_resource_0.directory which contains these
messages in a subfolder trash/new/ .

Prevously, yesterday, I had another Local Folders where the maildir folder was 
/home/freek/.local/share/0/akonadi_maildir_resource_0; I deleted that entry in
the configuration dialog of KMail.
However in ~/.config/kmail2rc there was still an entry: 
[Resource akonadi_maildir_resource_1], which should have been deleted by the
above action.

Searching in ~/.config for configuration files for KMail I did not find any
reference to this ~/.local/share/.akonadi_maildir_resource_0.directory .
File .config/akonadi_maildir_resource_0rc did contain the reference to
$HOME/.local/share/akonadi_maildir_resource_0


Quite confusing, consider it a bug.