Dovecot, Thunderbird and messages in wrong folder

[Gerhard Wiesinger]

Hello,

Sometimes it happens that messages (e.g. one or two) appear in wrong 
folders in Thunderbird (latest version, also past versions, different 
computers, also Earlybird). When I repair the folder 
(folder/Settings/Repair Folder) in Thunderbird, the folder is correct then.

It happens on my new server with Fedora 20, Dovecot 2.2.13 and Maildir 
but it also happened on my old server with Fedora 1, Dovecot 2.2.13 and 
mbox.

Has anyone the same issue?
Any guess whether it might be a thunderbird or dovecot issue?
Anyone has the same issues with thunderbird and a different IMAP server 
(e.g. cyrus)?

Thank you.

Ciao,
Gerhard

Config (only relevant parts here):
# 2.2.13: /etc/dovecot/dovecot.conf
# OS: Linux 3.15.8-200.fc20.x86_64 x86_64 Fedora release 20 (Heisenbug)
auth_mechanisms = plain login
lda_mailbox_autocreate = yes
listen = *
mail_gid = vmail
mail_home = /home/vmail/%n
mail_location = maildir:~/Maildir
mail_max_userip_connections = 20
mail_plugins = " stats mailbox_alias zlib"
mail_uid = vmail
protocols = imap lmtp sieve
service imap-login {
   inet_listener imap {
     port = 0
   }
   process_min_avail = 2
   service_count = 0
}
service imap {
   client_limit = 20
   executable = imap postlogin
   process_limit = 20
}
service lmtp {
   inet_listener lmtp {
     port = 24
   }
   process_min_avail = 2
}
service managesieve-login {
   process_min_avail = 1
}
service postlogin {
   executable = script-login -d rawlog
}
service stats {
   fifo_listener stats-mail {
     mode = 0600
     user = vmail
   }
}
ssl_cert = </etc/pki/dovecot/imap.crt
ssl_key = </etc/pki/dovecot/imap.key
submission_host = 127.0.0.1
userdb {
   args = username_format=%n /etc/dovecot/users
   driver = passwd-file
}
protocol imap {
   imap_client_workarounds = tb-extra-mailbox-sep
   mail_plugins = " stats mailbox_alias zlib imap_stats imap_zlib"
}
protocol lmtp {
   mail_plugins = " stats mailbox_alias zlib sieve"
}