dovecot - Jan 2017 - Panic: file istream-zlib.c: ...assertion failed: (ret == -1) / core dump on signal 6 / zlib

Hello!

I have several gzip'ed mbox Files in my home dir and the problem  
occurs when I use one of the gzip'ed files as shown mbox in the gui  
and want to see one mail of it.
It also happens on the extended search from Horde which then  such  a  
gzip'ed file.

Example file:
# file In_2014.gz
In_2014.gz: gzip compressed data, was "In_2014", last modified: Wed  
Dec  2 10:53:22 2015, from Unix

This happens with any gz Files. That's why I tried to sort out any  
problem else before submitting this.
Directly connecting to dovecot imap (circumvention imapproxy) shows  
the same error.

This setup was in place for 4 years with another server: It did not  
occur with dovecot 2.2.25 an up-imapproxy1.2.7 (Horde  
5.2.12/imp5.2.16) on FreeBsd9.3-p55 (my old server).
(I did not update to 2.2.27 on the last server as the maintainer just  
removed the ssl option from config I...and I knew I would update  
anyway soon,so I have no test of 2.2.27 with the old setup)

The actual setup as follows:
System is 10.3-RELEASE-p15 on amd64
Horde ist 5.2.13 / imp 6.2.17 / php-horde_lz4-1.0.7_1 (which is I  
guess unused here)
Lz4 is liblz4-131
FS is UFS

connection:
Horde webmail -> 1143:imapproxyd:143 -> dovceot imap  (or directly ->
same)

Jan 15 08:40:58 server dovecot: imap(xxx): Panic: file istream-zlib.c:  
line 416 (i_stream_zlib_seek): assertion failed: (ret == -1)
Jan 15 08:40:59 server kernel: pid 73887 (imap), uid 1000: exited on  
signal 6 (core dumped)
Jan 15 08:40:59 server dovecot: imap(xxx): Fatal: master:  
service(imap): child 73887 killed with signal 6 (core dumped)
Jan 15 08:40:59 server HORDE: [imp] [fetch] IMAP error reported by  
server. [pid 69574 on line 730 of  
"/usr/local/www/htdocs/m/imp/lib/Imap.php"]
Jan 15 08:40:59 server HORDE: [imp] [status] IMAP error reported by  
server. [pid 69574 on line 730 of  
"/usr/local/www/htdocs/m/imp/lib/Imap.php"]

and
Jan 15 08:40:56 server in.imapproxyd[912]: LOGOUT: 'xxx' from server sd
[12]
Jan 15 08:40:58 server in.imapproxyd[912]: LOGIN: 'xxx'  
(127.0.0.1:33269) on existing sd [12]
Jan 15 08:40:59 server in.imapproxyd[912]: Get_Server_conn(): Unable  
to reuse server sd [12] for user 'xxx' (127.0.0.1:50504).  Connection  
closed by server.

# dovecot -n
# 2.2.27 (c0f36b0): /usr/local/etc/dovecot/dovecot.conf
# OS: FreeBSD 10.3-RELEASE-p11 amd64
auth_mechanisms = plain login
disable_plaintext_auth = no
first_valid_gid = 100
first_valid_uid = 1000
mail_location = mbox:~/:INBOX=/var/mail/%u
mail_plugins = " zlib"
mail_privileged_group = mail
passdb {
    args = session=yes dovecot
    driver = pam
}
protocols = imap pop3
service auth {
    unix_listener auth-client {
      mode = 0660
    }
    unix_listener auth-master {
      mode = 0600
    }
    user = root
}
service imap {
    drop_priv_before_exec = yes
}
ssl = no
userdb {
    args = blocking=yes
    driver = passwd
}
verbose_proctitle = yes
protocol imap {
    imap_client_workarounds = delay-newmail tb-extra-mailbox-sep
}
protocol pop3 {
    pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
    pop3_uidl_format = %08Xu%08Xv
}
protocol lda {
    sendmail_path = /usr/sbin/sendmail
}

(gdb) bt full
#0  0x000000001115c39a in thr_kill () from /lib/libc.so.7
#1  0x000000001115c386 in raise () from /lib/libc.so.7
#2  0x000000001115c309 in abort () from /lib/libc.so.7
#3  0x0000000010e1c9b4 in default_fatal_handler () from  
/usr/local/lib/dovecot/libdovecot.so.0
#4  0x0000000010e1d6e1 in i_set_failure_internal () from  
/usr/local/lib/dovecot/libdovecot.so.0
#5  0x0000000010e1cc59 in i_panic () from  
/usr/local/lib/dovecot/libdovecot.so.0
#6  0x000000001144ffae in i_stream_create_deflate () from  
/usr/local/lib/dovecot/lib20_zlib_plugin.so
#7  0x0000000010e28051 in i_stream_seek_mark () from  
/usr/local/lib/dovecot/libdovecot.so.0
#8  0x0000000010ad7f15 in istream_raw_mbox_seek () from  
/usr/local/lib/dovecot/libdovecot-storage.so.0
#9  0x0000000010ad864d in mbox_file_seek () from  
/usr/local/lib/dovecot/libdovecot-storage.so.0
#10 0x0000000010ada7ad in mbox_dotlock_touch () from  
/usr/local/lib/dovecot/libdovecot-storage.so.0
#11 0x0000000010ada367 in mbox_dotlock_touch () from  
/usr/local/lib/dovecot/libdovecot-storage.so.0
#12 0x0000000010a9d365 in mail_get_hdr_stream_because () from  
/usr/local/lib/dovecot/libdovecot-storage.so.0
#13 0x0000000010b0cbe5 in index_mail_parse_headers () from  
/usr/local/lib/dovecot/libdovecot-storage.so.0
#14 0x0000000010b0f9d9 in index_mail_get_special () from  
/usr/local/lib/dovecot/libdovecot-storage.so.0
#15 0x0000000010b0f7b9 in index_mail_get_special () from  
/usr/local/lib/dovecot/libdovecot-storage.so.0
#16 0x0000000010a9d49f in mail_get_special () from  
/usr/local/lib/dovecot/libdovecot-storage.so.0
#17 0x000000000041cf1d in imap_fetch_handlers_deinit ()
#18 0x000000000041c561 in imap_fetch_more ()
#19 0x000000000041c1f8 in imap_fetch_more ()
#20 0x000000000040ff4e in cmd_fetch ()
#21 0x000000000041a4e6 in command_exec ()
#22 0x0000000000419573 in clients_destroy_all ()
#23 0x000000000041974f in clients_destroy_all ()
#24 0x0000000000418d9e in client_handle_input ()
#25 0x000000000041786c in client_input ()
#26 0x0000000010e31619 in io_loop_call_io () from  
/usr/local/lib/dovecot/libdovecot.so.0
#27 0x0000000010e3338d in io_loop_handler_run_internal () from  
/usr/local/lib/dovecot/libdovecot.so.0
#28 0x0000000010e31b33 in io_loop_handler_run () from  
/usr/local/lib/dovecot/libdovecot.so.0
#29 0x0000000010e31908 in io_loop_run () from  
/usr/local/lib/dovecot/libdovecot.so.0
#30 0x0000000010dbca08 in master_service_run () from  
/usr/local/lib/dovecot/libdovecot.so.0
#31 0x00000000004253f4 in main ()

Thanks
C.