dovecot - Dec 2017 - Panic: file imap-client.c: line 1204 (client_handle_input): assertion failed: (o_stream_is_corked(client->output))

Running Dovecot from the daily builds: 2:2.3.0~alpha0-1~auto+1287 in
an proxy setup:

auth_mechanisms = plain login
default_vsz_limit = 1 G
imapc_host = <redacted>.charite.de
imapc_port = 993
imapc_ssl = imaps
imapc_ssl_verify = no
listen = *,::
mail_gid = imapproxy
mail_home = /home/imapproxy/%u
mail_location = imapc:~/imapc
mail_plugins = mail_log notify
mail_uid = imapproxy
passdb {
  args = host=<redacted>.charite.de port=993 ssl=imaps
ssl_ca_file=/etc/ssl/certs/ca-certificates.crt
  default_fields = userdb_imapc_user=%u userdb_imapc_password=%w
userdb_imapc_host=<redacted>.charite.de userdb_imapc_ssl=imaps
userdb_imapc_port=993
  driver = imap
}
postmaster_address = postmaster at charite.de
protocols = imap
service auth {
  inet_listener {
    address = 127.0.0.1
    port = 12345
  }
}
ssl = required
ssl_ca = </etc/ssl/certs/ca-certificates.crt
ssl_cert = </etc/dovecot/dovecot.pem
ssl_cipher_list =
EECDH+ECDSA+AESGCM:EECDH+aRSA+AESGCM:EECDH+ECDSA+SHA384:EECDH+ECDSA+SHA256:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH+aRSA+RC4:EECDH:EDH+aRSA:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4
ssl_client_ca_file = /etc/ssl/certs/ca-certificates.crt
ssl_dh =  # hidden, use -P to show it
ssl_key =  # hidden, use -P to show it
ssl_prefer_server_ciphers = yes
userdb {
  driver = prefetch
}
verbose_proctitle = yes
  

Error in my log:

Dec 14 14:35:28 mproxy dovecot:
imap(voxxxxea)<79275><dnC4tUxgZs3BrwIS>: Panic: file imap-client.c:
line 1204 (client_handle_input): assertion failed:
(o_stream_is_corked(client->output))
Dec 14 14:35:28 mproxy dovecot:
imap(voxxxxea)<79305><taooukxgd83BrwIS>: Panic: file imap-client.c:
line 1204 (client_handle_input): assertion failed:
(o_stream_is_corked(client->output))
Dec 14 14:35:28 mproxy dovecot:
imap(voxxxxea)<79275><dnC4tUxgZs3BrwIS>: Error: Raw backtrace:
/usr/lib/dovecot/libdovecot.so.0(+0xc3cd5)
[0x7fe40d9cccd5] -> /usr/lib/dovecot/libdovecot.so.0(+0xc3dbc) 
[0x7fe40d9ccdbc] -> /usr/lib/dovecot/libdovecot.so.0(i_fatal+0) 
[0x7fe40d942f8e] -> dovecot/imap [voxxxxea
193.175.2.18](client_handle_input+0x2a8)
[0x564f326e0e18] -> dovecot/imap [voxxxxea
193.175.2.18](client_continue_pending_input+0x80)
[0x564f326e0f20] -> /usr/lib/dovecot/libdovecot.so.0(io_loop_call_io+0x69)
[0x7fe40d9e48f9] ->
/usr/lib/dovecot/libdovecot.so.0(io_loop_handler_run_internal+0x10a)
[0x7fe40d9e612a] ->
/usr/lib/dovecot/libdovecot.so.0(io_loop_handler_run+0x52)
[0x7fe40d9e4a02] -> /usr/lib/dovecot/libdovecot.so.0(io_loop_run+0x38)
[0x7fe40d9e4c18] -> /usr/lib/dovecot/libdovecot.so.0(master_service_run+0x13)
[0x7fe40d965b63] -> dovecot/imap [voxxxxea 193.175.2.18](main+0x333)
[0x564f326d3113] -> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf1)
[0x7fe40d5623f1] -> dovecot/imap [voxxxxea 193.175.2.18](+0xe2f1)
[0x564f326d32f1]
Dec 14 14:35:28 mproxy dovecot:
imap(voxxxxea)<79305><taooukxgd83BrwIS>: Error: Raw backtrace:
/usr/lib/dovecot/libdovecot.so.0(+0xc3cd5)
[0x7faa962d8cd5] -> /usr/lib/dovecot/libdovecot.so.0(+0xc3dbc) 
[0x7faa962d8dbc] -> /usr/lib/dovecot/libdovecot.so.0(i_fatal+0)
[0x7faa9624ef8e] -> dovecot/imap [voxxxxea
193.175.2.18](client_handle_input+0x2a8)
[0x5615124cde18] -> dovecot/imap [voxxxxea
193.175.2.18](client_continue_pending_input+0x80)
[0x5615124cdf20] -> /usr/lib/dovecot/libdovecot.so.0(io_loop_call_io+0x69)
[0x7faa962f08f9] ->
/usr/lib/dovecot/libdovecot.so.0(io_loop_handler_run_internal+0x10a)
[0x7faa962f212a] ->
/usr/lib/dovecot/libdovecot.so.0(io_loop_handler_run+0x52)
[0x7faa962f0a02] -> /usr/lib/dovecot/libdovecot.so.0(io_loop_run+0x38)
[0x7faa962f0c18] -> /usr/lib/dovecot/libdovecot.so.0(master_service_run+0x13)
[0x7faa96271b63] -> dovecot/imap [voxxxxea 193.175.2.18](main+0x333)
[0x5615124c0113] -> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf1)
[0x7faa95e6e3f1] -> dovecot/imap [voxxxxea 193.175.2.18](+0xe2f1)
[0x5615124c02f1]

-- 
Ralf Hildebrandt
  Gesch?ftsbereich IT | Abteilung Netzwerk
  Charit? - Universit?tsmedizin Berlin
  Campus Benjamin Franklin
  Hindenburgdamm 30 | D-12203 Berlin
  Tel. +49 30 450 570 155 | Fax: +49 30 450 570 962
  ralf.hildebrandt at charite.de | https://www.charite.de