auth: Error: BUG: Authentication client sent unknown handshake command

Hello!

I run dovecot for many years now, but today it ran into a bug I've never seen before. Searching the archives, this bug seems to occur very seldom.

The current server installation & configuration is running since mid 2015, uptime of the server today is more than two months. It is a virtual machine in a data center, debian 7 / reiserfs, 'dovecot -n' output below.

Just within normal operation, suddenly my users could no longer check their IMAP accounts, log shows lots of entries like this:

----------------------- /var/log/mail.log excerpt --------------------- 2017-01-08 08:15:23 dovecot: auth: Error: BUG: Authentication client sent unknown handshake command: REQUEST?3484549121?11364?1?74fa603b892f9a262605ae6c2ce1d9a1 2017-01-08 08:15:23 dovecot: imap: Error: Authentication server didn't send valid SPID as expected: MECH#011PLAIN#011plaintext 2017-01-08 08:15:23 dovecot: imap: Error: Disconnected from auth server, aborting (client-pid=11364 client-id=1) 2017-01-08 08:15:23 dovecot: imap-login: Internal login failure (pid=11364 id=1) (internal failure, 1 succesful auths): user=, method=PLAIN, rip=, lip=85.214.143.57, mpid=11368, TLS, session=<0oiwA5BFmQBQhoL1>

Strangely, sending mails still worked fine (postfix uses dovecot SASL authentification).

I could not find any hint what might have caused the error in any logfile. After restarting dovecot everything is back to normal operation.

Is there any other information I can / should provide?

Thanks for your efforts, Marcus

----------------------- output of 'dovecot -n' ------------------------ # 2.1.7: /etc/dovecot/dovecot.conf # OS: Linux 3.2.41-042stab120.16 i686 Debian 7.11 mail_location = maildir:~/Maildir 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 } prefix = } passdb { driver = pam } protocols = " imap" service auth { unix_listener /var/spool/postfix/private/auth { group = postfix mode = 0660 user = postfix } } ssl_cert =