Dovecot processes turning zombie

[Adrian POPA]

Hi

I am running Ubuntu 14.04 (plan to go to 16.04) with Postfix and Dovecot 2.2.9 from Ubuntu repository.
Dovecot configuration has not changed dramatically since the days of Ubuntu 12.04 (can't remember Dovecot version).
In the last months, connection to Dovecot IMAP service become more and more dificult. First Ubuntu Thunderbird refused to connect, than Android phones. The last one connecting regularly was an old XP laptop(!). Now it's facing problems too.
Again, I did not make any configuration changes lately so only system upgrades (libraries) may have caused this.
The first thing to notice was that some Dovecot processes are going zombie (defunct). At startup "ssl-params", then sometimes "auth", "imap-login" and rarely "config".
Turning on debug logging in Dovecot show that offending process "is ignoring idle SIGINT". And "auth client limit (1000) reached".
This behavior also impacts on Postfix delivery through Dovecot LDA with "status=deferred (temporary failure)" (I turned off mail sending with -e in lda invocation).
Another thing is SLL connection. Attempts to test with openssl s_client show that Dovecot is not responding after ClientHelo when it should normally send server certificate.
The very same certificate is used with Postfix and it works (I can send mails through Postfix) but IMAP connections to Dovecot do not. There is no protocol mismatch because I have try a local SSL connection (same computer). Initial messages are TLS1.2 but nothing in response.

So my question is: did you faced such problems and found a cause (and maybe a fix)? Upgrading to Ubuntu 16.04 with whatever Dovecot version it provides may resolve the issues?

Thanks