Error configuring unix_listener stats-writer

[Peter Nabbefeld via dovecot]

Hello,

I've got a very strange problem:
When I try to configure the stats-writer socket, I'm told the 
configuration is a duplicate, though none exists in the configuration 
files. So I'd guess the configuration is set either built-in or implicitly.

I've put the following into conf.d/10-master.conf:
service auth {
  unix_listener auth-userdb {
  }

  unix_listener stats-writer {
    user = vmail
    group = vmail
    mode = 0666
  }
}

BTW, using imap for mail traffic.

Is there any tool displaying an "effective" config tree (i.e., a tool 
displaying the resulting configuration from every file, displaying the 
original file and line number)? "dovecot -a" only displays all 
configurations, but without source reference, so it's impossible to find 
out about the details.  :-(

Kind regards

Peter