Mail folder "undefined" could not be found

No error before the previous reported, just informational messages :

INFO: Login:<user> IP:xx.xx.xx.xx AuthID:c1a8ae0914d144bdac4b8308d05abe23 Agent:Jakarta Commons-HttpClient/3.1 Client:null(null) Interface:HTTP_JSON Context:1(1,<customer>,defaultcontext) User:50(<user>) Session:1694e4d9f72047a2929fa90454dd53b5
22 oct. 2010 13:05:48 com.openexchange.login.internal.LoginPerformer logLoginRequest
INFO: Login:<user> IP:xx.xx.xx.xx AuthID:829ff1eed16a4bba80cc3cd51bd3a521 Agent:Jakarta Commons-HttpClient/3.1 Client:null(null) Interface:HTTP_JSON Context:1(1,<customer>,defaultcontext) User:50(<user>) Session:5b6556983c404e28856b144c0d3a80b2

[Viktor Pracht]

Are there any errors shortly before that error in the server log? Are there any errors in the error console in the browser?

Hello,

Our customer experiencing on many imap Dovecot users an error message in the OX webmail 6.18 (appears in /var/log/open-xchange/open-xchange.log.*) :

20 oct. 2010 16:10:18 com.openexchange.ajax.Mail actionGetAllMails
GRAVE: IMAP-1002 Category=8 Message=Mail folder "undefined" could not be found on server 127.0.0.1 with login <user> (user=61, context=1). exceptionID=-896783237-224153
IMAP-1002 Category=8 Message=Mail folder "undefined" could not be found on server 127.0.0.1 with login <user> (user=61, context=1). exceptionID=-896783237-224153
        at com.openexchange.imap.IMAPException.create(IMAPException.java:893)
        at com.openexchange.imap.IMAPException.create(IMAPException.java:857)
        at com.openexchange.imap.IMAPFolderWorker.setAndOpenFolder(IMAPFolderWorker.java:339)
        at com.openexchange.imap.IMAPMessageStorage.searchMessages(IMAPMessageStorage.java:383)
        at com.openexchange.mail.MailServletInterfaceImpl.getMessagesInternal(MailServletInterfaceImpl.java:1121)
        at com.openexchange.mail.MailServletInterfaceImpl.getMessages(MailServletInterfaceImpl.java:1111)
        at com.openexchange.mail.MailServletInterfaceImpl.getAllMessages(MailServletInterfaceImpl.java:598)
        at com.openexchange.ajax.Mail.actionGetAllMails(Mail.java:716)
        at com.openexchange.ajax.Mail.actionGetAllMails(Mail.java:591)
        at com.openexchange.ajax.request.MailRequest.action(MailRequest.java:115)
        at com.openexchange.ajax.Multiple.doAction(Multiple.java:293)
        at com.openexchange.ajax.Multiple.parseActionElement(Multiple.java:178)
        at com.openexchange.ajax.Multiple.doPut(Multiple.java:126)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:619)
        at com.openexchange.ajax.AJAXServlet.service(AJAXServlet.java:386)
        at com.openexchange.ajax.SessionServlet.service(SessionServlet.java:165)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
        at com.openexchange.ajp13.najp.AJPv13RequestHandlerImpl.doServletService(AJPv13RequestHandlerImpl.java:504)
        at com.openexchange.ajp13.AJPv13Request.response(AJPv13Request.java:129)
        at com.openexchange.ajp13.najp.AJPv13RequestHandlerImpl.createResponse(AJPv13RequestHandlerImpl.java:316)
        at com.openexchange.ajp13.najp.AJPv13ConnectionImpl.createResponse(AJPv13ConnectionImpl.java:207)
        at com.openexchange.ajp13.najp.AJPv13Task.call(AJPv13Task.java:346)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
        at java.util.concurrent.FutureTask.run(FutureTask.java:138)
        at com.openexchange.threadpool.internal.CustomThreadPoolExecutor$Worker.runTask(CustomThreadPoolExecutor.java:737)
        at com.openexchange.threadpool.internal.CustomThreadPoolExecutor$Worker.run(CustomThreadPoolExecutor.java:763)
        at java.lang.Thread.run(Thread.java:619)

The error message disappear when clicking on the Inbox.
I think the problem is on Dovecot side, but full debug don't give any clue for the moment.

Thanks in advance for any other clues to explore.