BUG: Authentication client sent unknown handshake command

[Lukas Haase]

Hi,

Some time ago my dovecot started to become BUGgy: Evertime I restart the
system, dovecot is started but I cannot login. I see the following
information in the logs:

Jul  4 04:04:36 mail dovecot: auth:
passwd-file(user,127.11.91.218,<nfkQjFT9vwCADFra>): no passwd file:
/etc/dovecot/imap.passwd
Jul  4 04:04:36 mail dovecot: auth: Error: BUG: Authentication client
sent unknown handshake command:
REQUEST?1183186945?20438?1?a8c7241e7b0778f9f0125da760cd16fa
Jul  4 04:04:36 mail dovecot: imap: Error: Authentication server didn't
send valid SPID as expected: MECH#011PLAIN#011plaintext
Jul  4 04:04:36 mail dovecot: imap: Error: Disconnected from auth
server, aborting (client-pid=20438 client-id=1)
Jul  4 04:04:36 mail dovecot: imap-login: Internal login failure
(pid=20438 id=1) (internal failure, 1 succesful auths): user=<user>,
method=PLAIN, rip=127.11.91.218, lip=73.82.101.172, mpid=20439, TLS,
session=<nfkQjFT9vwCADFra>

It does not help to restart the client. But when I then restart the server
  service dovecot restart
it works again.

But this is very dangerous because the server might reboot and I might
not immideately have the chance to restart dovecot manually.

What's going on here?

mail:/# uname -a
Linux mail 2.6.32-openvz-042stab090.2-amd64 #1 SMP Sat Jun 21 00:22:42
MSK 2014 i686 GNU/Linux
mail:/# dovecot --version
2.1.7

Thanks,
Luke