dovecot - Oct 2015 - Dovecot 2.2.19 - Panic: file dict-sql.c: line 670 (sql_dict_iterate): assertion failed: ((ctx->flags & DICT_ITERATE_FLAG_ASYNC) != 0)

Hello everyone,
For this installation I am using passdb against AD, userdb with MySQL and nfs
storage but using simultaneously the vfile acl backend and the shared mailbox
dictionary I get the following panic:
Oct 19 12:34:51 server dovecot: imap-login: Login: user=<user04>,
method=PLAIN, rip=10.112.99.52, lip=10.113.63.50, mpid=7538,
session=<NKsDsnIiU7YKcGM0>
Oct 19 12:34:51 server dovecot: dict(7540): Panic: file dict-sql.c: line 670
(sql_dict_iterate): assertion failed: ((ctx->flags &
DICT_ITERATE_FLAG_ASYNC) != 0)
Oct 19 12:34:51 server dovecot: dict(7540): Error: Raw backtrace:
/usr/lib64/dovecot/libdovecot.so.0() [0x31826800ca] ->
/usr/lib64/dovecot/libdovecot.so.0() [0x3182680136] ->
/usr/lib64/dovecot/libdovecot.so.0() [0x31826299ea] -> dovecot-mailbox/dict()
[0x406abc] -> dovecot-mailbox/dict() [0x4047aa] -> dovecot-mailbox/dict()
[0x404f32] -> dovecot-mailbox/dict(dict_command_input+0xab) [0x4050bb] ->
dovecot-mailbox/dict() [0x40425b] ->
/usr/lib64/dovecot/libdovecot.so.0(io_loop_call_io+0x5b) [0x3182692deb] ->
/usr/lib64/dovecot/libdovecot.so.0(io_loop_handler_run_internal+0xb7)
[0x31826943c7] ->
/usr/lib64/dovecot/libdovecot.so.0(io_loop_handler_run+0x25) [0x3182692e95]
-> /usr/lib64/dovecot/libdovecot.so.0(io_loop_run+0x38) [0x3182693038] ->
/usr/lib64/dovecot/libdovecot.so.0(master_service_run+0x13) [0x318262e953] ->
dovecot-mailbox/dict(main+0x165) [0x405295] ->
/lib64/libc.so.6(__libc_start_main+0xfd) [0x303be1ed5d] ->
dovecot-mailbox/dict() [0x403da9]
Oct 19 12:34:51 server dovecot: dict(7540): Fatal: master: service(dict): child
7540 killed with signal 6 (core dumps disabled)
am I missing something obvious or this is a dovecot bug?

# 2.2.19: /etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.9 (357ac0a0e68b+)
# OS: Linux 2.6.32-573.1.1.el6.x86_64 x86_64 Red Hat Enterprise Linux Server
release 6.7 (Santiago)
auth_master_user_separator = *
auth_username_format = %Ln
dict {
? acl = mysql:/etc/dovecot/dovecot-dict-acl.conf.ext
}
disable_plaintext_auth = no
instance_name = dovecot-mailbox
lda_mailbox_autocreate = yes
lda_mailbox_autosubscribe = yes
listen = *
lmtp_rcpt_check_quota = yes
lmtp_save_to_detail_mailbox = yes
mail_fsync = always
mail_gid = vmail
mail_nfs_index = yes
mail_nfs_storage = yes
mail_plugins = " mail_log notify acl mailbox_alias quota listescape"
mail_uid = vmail
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope encoded-character
vacation subaddress comparator-i;ascii-numeric relational regex im
ap4flags copy include variables body enotify environment mailbox date index
ihave duplicate imapflags notify
mbox_write_locks = fcntl
mmap_disable = yes
namespace {
? list = children
? location = maildir:%%h/Mail:INDEX=~/shared/%%u:INDEXPVT=~/shared-pvt/%%u
? prefix = shared/%%u/
? separator = /
? subscriptions = no
? type = shared
}
namespace inbox {
? inbox = yes
? location = 
? mailbox Drafts {
??? special_use = \Drafts
? }
? mailbox Junk {
??? special_use = \Junk
? }
? mailbox Sent {
??? special_use = \Sent
? }
? mailbox "Sent Messages" {
??? special_use = \Sent
? }
? mailbox Trash {
??? special_use = \Trash
? }
? mailbox sent_mail_folder {
??? special_use = \Sent
? }
? mailbox trash_folder {
??? special_use = \Trash
? }
? prefix = 
? separator = /
? type = private
}
passdb {
? args = /etc/dovecot/dovecot-pass_db-ldap.conf.ext
? driver = ldap
}
plugin {
? acl = vfile
? acl_shared_dict = proxy::acl
? mail_log_events = delete undelete expunge copy mailbox_delete mailbox_rename
flag_change append
? mail_log_fields = uid box msgid from subject size flags
? sieve = file:~/sieve;active=~/.dovecot.sieve
? sieve_extensions = +notify +imapflags
? sieve_max_actions = 32
? sieve_max_redirects = 25
? sieve_max_script_size = 1M
? sieve_quota_max_storage = 10M
}
protocols = imap pop3 lmtp sieve
service auth {
? unix_listener auth-userdb {
??? group = vmail
? }
}
service dict {
? unix_listener dict {
??? group = dovecot
??? mode = 0660
??? user = vmail
? }
}
service lmtp {
? inet_listener lmtp {
??? port = 24
? }
}
service managesieve-login {
? inet_listener sieve {
??? port = 4190
? }
? inet_listener sieve_deprecated {
??? port = 2000
? }
? service_count = 1
}
service managesieve {
? process_limit = 1024
}
ssl = no
userdb {
? args = /etc/dovecot/dovecot-user_db-sql.conf.ext
? driver = sql
}
verbose_proctitle = yes
protocol lmtp {
? mail_plugins = " mail_log notify acl mailbox_alias quota listescape
sieve"
}
protocol lda {
? mail_plugins = " mail_log notify acl mailbox_alias quota listescape
sieve"
}
protocol imap {
? mail_plugins = " mail_log notify acl mailbox_alias quota listescape
imap_acl imap_quota"
}
protocol sieve {
? managesieve_implementation_string = Dovecot Pigeonhole
? managesieve_logout_format = bytes=%i/%o
? managesieve_max_compile_errors = 1
? managesieve_max_line_length = 65536
}
protocol pop3 {
? mail_plugins = " mail_log notify acl mailbox_alias quota listescape"
}