On a clean Debian 9 machine, I am trying to install Froxlor, using the following commands:

echo 'deb http://debian.froxlor.org stretch main' >> /etc/apt/sources.list.d/froxlor.list
apt install -y dirmngr
mkdir -p ~/.gnupg ; echo 'disable-ipv6' >> ~/.gnupg/dirmngr.conf
apt-key adv --keyserver pool.sks-keyservers.net --recv-key FD88018B6F2D5390D051343FF6B4A8704F9E9BBC
apt-get update ; apt-get -y upgrade
apt-get install -y froxlor

Everything seems t be progressing OK, but at some point during the installation I get the following error:

Processing triggers for dovecot-core (1:2.2.27-3+deb9u2) ...
Job for dovecot.service failed because the control process exited with error code.
See "systemctl status dovecot.service" and "journalctl -xe" for details.
invoke-rc.d: initscript dovecot, action "restart" failed.
● dovecot.service - Dovecot IMAP/POP3 email server
   Loaded: loaded (/lib/systemd/system/dovecot.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Fri 2018-04-27 19:03:42 EEST; 8ms ago
     Docs: man:dovecot(1)
           http://wiki2.dovecot.org/
  Process: 19510 ExecStop=/usr/bin/doveadm stop (code=exited, status=0/SUCCESS)
  Process: 19515 ExecStart=/usr/sbin/dovecot (code=exited, status=89)
 Main PID: 15281 (code=exited, status=0/SUCCESS)

Apr 27 19:03:42 lamp dovecot[19515]: Error: socket() failed: Address family not supported by protocol
Apr 27 19:03:42 lamp dovecot[19515]: master: Error: socket() failed: Address family not supported by protocol
Apr 27 19:03:42 lamp dovecot[19515]: Error: service(imap-login): listen(::, 143) failed: Address family not supported by protocol
Apr 27 19:03:42 lamp dovecot[19515]: master: Error: service(imap-login): listen(::, 143) failed: Address family not supported by protocol
Apr 27 19:03:42 lamp dovecot[19515]: Fatal: Failed to start listeners
Apr 27 19:03:42 lamp dovecot[19515]: master: Fatal: Failed to start listeners
Apr 27 19:03:42 lamp systemd[1]: dovecot.service: Control process exited, code=exited status=89
Apr 27 19:03:42 lamp systemd[1]: Failed to start Dovecot IMAP/POP3 email server.
Apr 27 19:03:42 lamp systemd[1]: dovecot.service: Unit entered failed state.
Apr 27 19:03:42 lamp systemd[1]: dovecot.service: Failed with result 'exit-code'.
dpkg: error processing package dovecot-core (--configure):
 subprocess installed post-installation script returned error exit status 1
Processing triggers for libapache2-mod-php7.0 (7.0.27-0+deb9u1) ...
Errors were encountered while processing:
 dovecot-core

Attached is the full log
 

log.txt

The froxlor Debian package just adds dovecot as dependency, why Debian/APT fails to Install it correctly is not related to froxlor.

As it seems to be a configuration related Problem, have you tried configuring your system with the froxlor config-templates and checked whether the issue persists?

it seems the issue was that my machine had ipv6 disabled. the problem is solved now.