[Dovecot] Dovecot doesn't seem to report all subfolders to Thunderbird

[dovecot at voidptr.de]

Hi,

I've recently migrated my emails from a Yahoo account to my own
mailserver on a VPS.

Ever since then, Thunderbird seems to have issues with the mailbox's
directory structure: When I start Thunderbird, it immediately deletes
the local copies of all 3rd-level folders (i.e. subfolders of a
subfolder of a folder in the mailbox root).
Since that didn't happen with the Yahoo account, I assume it's some kind
of error or misconfiguration of Dovecot.

Here's an example of the mailbox structure:
voidptr.de
'-> University
    '-> IEEE
        '-> Conference
"University" and "IEEE" work without problems, but "Conference" gets
deleted locally every time I start Thunderbird. When I collapse and
re-expand "University" in the folder panel, all subfolders (including
"Conference") reappear, but Thunderbird has to re-download the contents
of "Conference", since it deleted the local copies earlier.

It looks as if Dovecot only reported the topmost two folder levels when
Thunderbird requests a folder list...

Is there any way I can change that behavior?

Nils



$ doveconf -n
# 2.1.7: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-042stab078.22 x86_64 Debian 7.3
auth_verbose = yes
mail_location = maildir:~/Maildir:LAYOUT=fs
namespace {
  inbox = yes
  location =
  mailbox Drafts {
    auto = subscribe
    special_use = \Drafts
  }
  mailbox Junk {
    auto = subscribe
    special_use = \Junk
  }
  mailbox Sent {
    auto = subscribe
    special_use = \Sent
  }
  mailbox Trash {
    auto = subscribe
    special_use = \Trash
  }
  prefix =
  subscriptions = yes
}
passdb {
  args = scheme=SHA512-CRYPT username_format=%n /etc/dovecot/users
  driver = passwd-file
}
protocols = " imap"
service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0666
  }
}
service imap-login {
  inet_listener imaps {
    port = 0
  }
}
ssl = required
ssl_cert = </etc/ssl/certs/own/voidptr.de.pem
ssl_key = </etc/ssl/certs/own/voidptr.de.key
userdb {
  args = username_format=%n /etc/dovecot/users
  default_fields = home=/home/%n
  driver = passwd-file
}