Dovecot fails to start after upgrade to Plesk 17.8 on Proxmox: Failed at step NAMESPACE spawning /usr/bin/doveadm: Invalid argument

Symptoms

1. Apparmor is not installed:

   # aa-status
   apparmor module is not loaded.

2. Plesk is running on Debian/Ubuntu in Proxmox virtual environment: 

   # plesk bin server_pref --get-server-info 2>&1 | grep virtual
   "virtualization": "lxc",

3. Dovecot fails to start after upgrade to Plesk 17.8: 

   Active: failed (Result: exit-code) since Mon 2018-04-23 11:59:33 CEST; 2s ago
   Docs: man:dovecot(1)
   http://wiki2.dovecot.org/
   Process: 11856 ExecStop=/usr/bin/doveadm stop (code=exited, status=226/NAMESPACE)
   Process: 11855 ExecStart=/usr/sbin/dovecot -F (code=exited, status=226/NAMESPACE)

4. The following error is logged to journald:

   # journalctl -f
   systemd[11361]: dovecot.service: Failed at step NAMESPACE spawning /usr/bin/doveadm: Invalid argument

Cause

Unclear.

Resolution

1. Connect to Plesk server via SSH

2. Create the directory for drop-in configuration:

   # mkdir /lib/systemd/system/dovecot.service.d/

3. Create /lib/systemd/system/dovecot.service.d/pleskfix.conf file with the override configuration:

   [Service]
   PrivateTmp=false
   ProtectSystem=false
   PrivateDevices=false
   NoNewPrivileges=false

4. Reload systemD services:

   # systemctl daemon-reload

5. Start the Dovecot service:

   # systemctl start dovecot

Additional information

If Apparmor is used, check this article.