Strange SASL issue

Hi all,

I've just upgraded my server from debian 8 to debian 9 and I now encounter a strange issue. I'm using prosody (XMPP server), configured to authenticate against dovecot using SASL and a unix socket.

This setup was working fine on debian 8.

Now, each time I try to connect using my XMPP client, the authentication fails and I see the following error in logs:

auth: Error: BUG: Authentication client said it's PID 0

Have you ever heard about this?

For information, dovecot version is 2.2.27.

Thank you, Antoine Nguyen