Domain on Plesk with WHMCS shows error: The PHP session save path /var/lib/php/session is not writable

Symptoms

• Domain with WHMCS shows error:

  PLESK_INFO: The PHP session save path /var/lib/php/session is not writable. Please investigate the session.save_path PHP setting or contact your system administrator.

  ---

  PLESK_INFO: Warning: session_start(): open(C:\Windows\Temp\sess_udfqa0si9tsg9i89dhene7ms78, O_RDWR) failed: Permission denied (13) in C:\inetpub\vhosts\example.com\includes\connect.php on line 2

• Permissions for the folder mentioned in the error are correct:

  # ls -la /var/lib/php/
  ...
  drwx-wx-wt 2 root root 4096 Nov 15 23:36 session

  ---

  C:\> \Users\Administrator>icacls C:\Windows\Temp
  C:\Windows\Temp\ NT AUTHORITY\NETWORK SERVICE:(CI)(RX,D,WD,AD)
  HOSTNAME\PP_WPG:(CI)(RX)
  HOSTNAME\psaserv:(CI)(RX)
  HOSTNAME\psacln:(CI)(RX)
  HOSTNAME\psaadm:(CI)(RX)
  HOSTNAME\psaadm:(OI)(IO)(R)
  BUILTIN\Users:(CI)(S,WD,AD,X)
  BUILTIN\IIS_IUSRS:(OI)(CI)(S,RD)
  CREATOR OWNER:(OI)(CI)(IO)(F)
  NT AUTHORITY\SYSTEM:(OI)(CI)(F)
  BUILTIN\Administrators:(OI)(CI)(F)

Cause

Subscription system user directory for sessions is not created or it is not set in the session.save_path PHP settings.

Resolution

Note: in case there is no WHMCS installed on the same server, refer to the following article: Website hosted in Plesk displayed incorrectly or showing the PHP warning: Failed to read session data: files

Create a special directory for the PHP sessions: