Thunderbird ignores some folders

[dovecot.pkoch at dfgh.net]

Dear readers

we are using Dovecot 2.2.7 and all of our users are using Thunderbird as
their mail client. Some of them additionally use their iPad/iPhone and a
very few an Android Mail-Client.

Now one user noticed that two of his mail folders disappeared. He first
believed that he accidentally deleted those folders but then he realized
that they are still visible from his iPad. I checked this users maildir and
everything looks normal. Deleting the index-files made no difference: New
index-files were created but those two folders are still invisable from
Thunderbird.

My main concern is to find the reason for this behaviour. Maybe folders of
other users have disappeared and they have not yet noticed. A fresh
installation of Thunderbird will most likely fix this single users problem,
but I cannot do this for all of our users.

So here's my question: How would I debug such a situation?

Kind regards

Peter Koch

And here's our config file:

# Dovecot configuration file

default_process_limit = 2048

protocols = imap
listen    = *
base_dir  = /var/dovecot/

mail_location = maildir:/mail/%u:LAYOUT=fs

ssl_cert = </etc/ssl/certs/naev+chain.crt
ssl_key  = </etc/ssl/private/naev.key
ssl_ca   = </etc/ssl/certs/naev-ca.crt
ssl_verify_client_cert  = yes
ssl_cert_username_field = x500UniqueIdentifier
auth_ssl_username_from_cert = yes

service imap-login {
  inet_listener imap {
    port = 0
  }
}

service auth {
  client_limit = 4096
}

service anvil {
  client_limit = 5000
}

passdb {
  driver = checkpassword
  args   = /usr/dovecot/bin/checkpassword
}

userdb {
  driver          = passwd
  override_fields = home=/mail/%u
}

protocol imap {
   mail_max_userip_connections = 1000
}