Disconnected in APPEND depends on service_count?

[Hajo Locke]

Hello,

we have quite odd behaviour of dovecot under new OS.
We use dovecot 2.2.18 for years under Ubuntu12 and Ubuntu14. Now we use 
Ubuntu16 and notice some strange errors.
dovecot 2.2.18 is rather old, but runs reliable and we dont need newest 
features, so we packaged it again on Ubuntu16.
For explanation: We use some own patches for dovecot and insert some 
functions in deb packages, so we want to reuse our code to minimize effort.

Under ubuntu16 we see a lot of messages like this on high load servers:

Aug 22 08:28:15 hostname dovecot: imap(username): Disconnected in APPEND 
(1 msgs, 0 secs, 8192/136342 bytes) in=8379 out=830
Aug 22 08:28:16 hostname dovecot: imap(username): Disconnected in APPEND 
(1 msgs, 0 secs, 8192/136342 bytes) in=8985 out=2931
Aug 22 08:42:58 hostname dovecot: imap(username): Disconnected in APPEND 
(1 msgs, 29 secs, 13139969/42801634 bytes) in=13140054 out=404
Aug 22 08:43:01 hostname dovecot: imap(username): Disconnected in APPEND 
(1 msgs, 3 secs, 1286145/42801634 bytes) in=1286230 out=404
Aug 22 08:43:29 hostname dovecot: imap(username): Disconnected in APPEND 
(1 msgs, 2 secs, 704476/42801634 bytes) in=704561 out=404
Aug 22 08:44:29 hostname dovecot: imap(username): Disconnected in APPEND 
(1 msgs, 60 secs, 27402241/42801634 bytes) in=27402326 out=404
Aug 22 08:45:30 hostname dovecot: imap(username): Disconnected in APPEND 
(1 msgs, 3 secs, 1458177/42801634 bytes) in=1458262 out=404
Aug 22 08:46:15 hostname dovecot: imap(username): Disconnected in APPEND 
(1 msgs, 45 secs, 20594652/42801634 bytes) in=20594737 out=404
Aug 22 08:47:38 hostname dovecot: imap(username): Disconnected in APPEND 
(1 msgs, 11 secs, 4907009/42801634 bytes) in=4907094 out=404

Error occured after sending a mail, when client tries to save sent mail 
in sent-messages folder. (mbox)
Typically this is a client problem caused by firewall/virusguard or 
network erros. But we saw that many different users are effected and we 
could reproduce this behaviour on clean clients. In our logs and systemd 
journal we had no hints leading to other error-messages, no dovecot 
message which tells that limit xy was reached.  Also no hints for other 
malfunctioning services. No helpful lines with mail_debug=yes
We tried a lot and it seems that changing "service_count = 0"  to 
"service_count = 1" is part of solution. After changing and restarting 
the behaviour is like expected.

old:
service imap-login {
   executable = /usr/lib/dovecot/imap-login
   process_min_avail = 1
   service_count = 0
   user = dovecot
}

new:

service imap-login {
   executable = /usr/lib/dovecot/imap-login
   process_min_avail = 1
   service_count = 1
   user = dovecot
   process_limit = 10000
}

service_count switches between number of handled client-connections. i 
dont see a relation between service_count and the permanent disconnects 
but it seems to be part of problem.
Can anybody explain this behaviour or give some handy tips?

Thanks,
Hajo