[Dovecot] Thunderbird does not display old IMAP messages

Hi

I have a strange trouble with Dovecot 1.1.10 and Thnuderbird 2.0.0.19

it happens that when a user starts its thunderbird client, he is unable to see his mailbox subject ( the mailbox appears empty )

If he receive a NEW email, then ALL the mailbox contents appears.

Any idea on what happen ( cache problem ? index problem ? )

Thanks