[Dovecot] Error: Threading lost Message ID?

[Ralf Hildebrandt]

QUERY:    THREAD REFERENCES ISO-8859-1 ALL
RESPONSE: [SERVERBUG] Internal error occurred. Refer to server log for more information. [2013-08-09 15:42:34]

Aug  9 15:41:48 postamt dovecot: imap(username) tTbs7oPjfACNKs4m: Error: Threading lost Message ID
Aug  9 15:42:34 postamt dovecot: imap(username) tTbs7oPjfACNKs4m: Error: Threading lost Message ID

What does that mean? WHY has the Message-Id been lost? How can I find
out WHICH MAIL caused this?

This is the most recent dovecot 2.2.x

-- 
Ralf Hildebrandt
  Geschäftsbereich IT | Abteilung Netzwerk
  Charité - Universitätsmedizin Berlin
  Campus Benjamin Franklin
  Hindenburgdamm 30 | D-12203 Berlin
  Tel. +49 30 450 570 155 | Fax: +49 30 450 570 962
  ralf.hildebrandt at charite.de | http://www.charite.de