[Dovecot] Timeout while waiting for shared lock for index file /var/mail/XXX/Maildir/dovecot.index

Hello,

I'm using dovecot v1.0rc23 for about 500 users since february with no particular problems. I'm using Maildir format and ldap authentication. A recently added user can't check his mails any more. In the logs I get

May 23 14:35:58 mailsrv deliver(XXXX): [ID 702911 mail.error] Timeout while waiting for shared lock for index file /var/mail/XXXX/Maildir/dovecot.index

The user connects with IMAP to retreive his mails. Currently I can login to his account with pop3 and retreive the mails, but when it comes to IMAP I get this error and no mail at all.

thanks in advance,

giannis.