IMAP Folders feature not working

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When pressing the "IMAP Folders" button in the E-Mail preflet, I am presented with an empty window with another window in front of it requesting patience. :-) The mail_daemon will immediately peek on of the CPUs (the IMAP inbox thread). There seems to be no way to stop the operation other than killing the preflet. In one of my tries, the patience window disappeared and the IMAP Foldes window showed an empty list (I assume it's a list, it doesn't have a border, just a white square). After pressing the Apply button, the window closed, but the CPU usage of mail_daemon persisted.