Unable to install WordPress in Plesk: move_to_cgroup(): pam_open_session() failed 6: Permission denied

Symptoms

• The following error occurs on an attempt to install WordPress via WordPress Toolkit in Plesk:

  filemng failed: move_to_cgroup(): pam_open_session() failed 6: Permission denied
  move process 15666 to cgroup for user plesktest failed
  filemng: Error occurred during /bin/mkdir command.

Cause

This is the systemd bug. "systemd. systemd-logind" takes ~25sec timeout after ~1000 ssh logins. During these 25 seconds, the resctrl utility is doing its own maintenance and gets a dbus timeout.
Additional information about the bug is available in these articles:

• https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1591411
• https://bugs.freedesktop.org/show_bug.cgi?id=95264.

Resolution

As a workaround, change dbus timeout for SSHD service as follows:

1. Connect to the server via SSH;

2. Create a dbus config with the following content setting timeout to 5 seconds:

   # cat /etc/dbus-1/session-local.conf
   <busconfig>
   <limit name="auth_timeout">50000</limit>
   </busconfig>

3. Reload required services:

   # service dbus restart
   # systemctl daemon-reexec

Note: as this workaround has DDoS attack risk, do not forget to change the SSH port as it is described in the following article:
How to change the SSH port number on a Linux server?