Error: Filename keeps changing

[Ward Romp]

Dear all,

Two days ago, my ldap server (for authentication) crashed and I had to 
set up a new one. Everything works fine except the mailbox for one user 
(let's name him debugger). When I try to access his mail whatever way, 
dovecot complains about "Error: maildir_file_do(........): Filename 
keeps changing"

For example, a command "doveadm copy -u debugger at boontje.net Archive 
mailbox Netspar" returns
doveadm(debugger at boontje.net): Error: 
maildir_file_do(/var/mail/vmail/boontje.net/debugger/mail/Netspar/cur/1423052681.M610997P2829.mail,S=1499407,W=1518981:2,S): 
Filename keeps changing
doveadm(debugger at boontje.net): Error: Copying message UID 1 from 
'Netspar' failed: Internal error occurred. Refer to server log for more 
information. [2016-05-24 13:19:58]
doveadm(debugger at boontje.net): Error: 
maildir_file_do(/var/mail/vmail/boontje.net/debugger/mail/Netspar/cur/1447239217.M843201P13440.mail,S=6052999,W=6131730:2,RSa): 
Filename keeps changing
doveadm(debugger at boontje.net): Error: Copying message UID 2 from 
'Netspar' failed: Internal error occurred. Refer to server log for more 
information. [2016-05-24 13:19:58]
doveadm(debugger at boontje.net): Error: 
maildir_file_do(/var/mail/vmail/boontje.net/debugger/mail/Netspar/cur/1447239226.M993616P13440.mail,S=13920,W=14288:2,S): 
Filename keeps changing
doveadm(debugger at boontje.net): Error: Copying message UID 3 from 
'Netspar' failed: Internal error occurred. Refer to server log for more 
information. [2016-05-24 13:19:58]
doveadm(debugger at boontje.net): Error: 
maildir_file_do(/var/mail/vmail/boontje.net/debugger/mail/Netspar/cur/1449826302.M51386P11767.mail,S=1252017,W=1268393:2,RS): 
Filename keeps changing
doveadm(debugger at boontje.net): Error: Copying message UID 4 from 
'Netspar' failed: Internal error occurred. Refer to server log for more 
information. [2016-05-24 13:19:58]

It does this on ALL mailboxes of this user and only this user. Other 
users give no problems. Accessing the mails via a mail client 
(Roundcube, Thunderbird, etc...) give the same error.

Dovecot runs on a virtual machine and the mailboxes are stored via a 
CIFS-share on a NAS. I rebooted both machines, checked the filesystem 
(no errors), CIFS-share, directories and mail files are readable and 
writeable by dovecot user (vmail on the mail server). The mails are 
intact, cat <filename> shows the full mail.

I'm baffled, any suggestions?

Kind regards,

Ward Romp


The configuration etc....
================================================

# 2.2.9: /etc/dovecot/dovecot.conf
# OS: Linux 4.2.0-36-generic x86_64 Ubuntu 14.04.4 LTS cifs
auth_mechanisms = plain login
auth_verbose = yes
listen = *
mail_home = /var/mail/vmail/%d/%n
mail_location = maildir:/var/mail/vmail/%d/%n/mail:LAYOUT=fs
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope 
encoded-character vacation subaddress comparator-i;ascii-numeric 
relational regex imap4flags copy include variables body enotify 
environment mailbox date ihave
namespace inbox {
   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 =
}
passdb {
   args = /etc/dovecot/conf.d/ldap.conf.ext
   driver = ldap
}
plugin {
   sieve = ~/.dovecot.sieve
   sieve_after = /var/mail/vmail/sieve-after
   sieve_before = /var/mail/vmail/sieve-before
   sieve_dir = ~/sieve
}
protocols = imap sieve
service auth {
   unix_listener /var/spool/postfix/private/dovecot-auth {
     group = postfix
     mode = 0660
     user = postfix
   }
}
ssl_cert = </etc/ssl/private/mail.boontje.net-chain.crt
ssl_cipher_list = 
ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+3DES:DH+3DES:RSA+AES:RSA+3DES:!ADH:!AECDH:!MD5:!DSS
ssl_client_ca_dir = /etc/ssl/certs
ssl_key = </etc/ssl/private/mail.boontje.net.key
userdb {
   args = uid=5000 gid=5000 home=/var/mail/vmail/%d/%n allow_all_users=yes
   driver = static
}
protocol imap {
   imap_client_workarounds = delay-newmail tb-extra-mailbox-sep
   mail_max_userip_connections = 10
}
protocol lda {
   deliver_log_format = msgid=%m: %$
   mail_plugins = sieve
   postmaster_address = webmaster at boontje.net
   quota_full_tempfail = yes
   rejection_reason = Your message to <%t> was automatically rejected:%n%r