Strange "IMAP connection broken (server response)" errors

[Jozsef Kadlecsik]

Hello,

We upgraded one of our dovecot servers to debian stretch with dovecot 
2.2.27 and since then one of our users has been experiencing random IMAP 
failures.

We enabled raw logging at the server side and it shows normal IMAP 
commands/responses:

1507292522.222427 * 6 FETCH (FLAGS () BODYSTRUCTURE ("text" "plain" 
("charset" "us-ascii")
 NIL NIL "7bit" 4645 112 NIL NIL NIL NIL))
1507292522.222653 00000011 OK Fetch completed (0.006 + 0.000 secs).

On the client side the user runs alpine and the corresponding debug lines:

IMAP DEBUG 14:22:02.216167: 00000011 FETCH 6 (BODYSTRUCTURE FLAGS)

14:22:02.217396
IMAP 14:22:02 10/6 mm_notify bye: 
{[127.0.0.1]:1555/imap/user="ha4aa"}INBOX: [CLOSED] IMAP connection broken 
(server response)

14:22:02.217471
IMAP 14:22:02 10/6 mm_log error: [CLOSED] IMAP connection broken (server 
response)

The "[127.0.0.1]:1555/imap/user="ha4aa" part in the log comes from an 
socat inserted between the client and the server to check independetly the 
imap session. According to socat, the server response didn't reach the 
client!:

00000010 OK Fetch completed (0.005 + 0.000 secs).\r
> 2017/10/06 14:22:02.216299  length=40 from=845 to=884
00000011 FETCH 6 (BODYSTRUCTURE FLAGS)\r

and here ends the socat log, the server response didn't reach the client.

The same alpine package is used against older dovecot servers from debian 
jessie and works flawlessly.

The server and client are both KVM guests and there's no any content 
filtering software between them. There's no corresponding error in the 
dovecot log file and dovecot didn't regard the connection broken and 
detected client close:

...
Oct  6 14:19:44 mail2 dovecot: imap(ha4aa): Debug: Mailbox INBOX: Opened 
mail UID=55244 because: MIME part
Oct  6 14:22:02 mail2 dovecot: imap(ha4aa): Connection closed in=797 
out=73151

We are out of the ideas what can be the reason for the random lost server 
responses.

Any help is appreciated!

Best regards,
Jozsef
--
E-mail : kadlecsik.jozsef at wigner.mta.hu
PGP key: http://www.kfki.hu/~kadlec/pgp_public_key.txt
Address: Wigner Research Centre for Physics, Hungarian Academy of Sciences
         H-1525 Budapest 114, POB. 49, Hungary