Plesk shows 503 Service Unavailable: iisnode.dll failed to load The data is the error

Applicable to:

When loading Plesk the following error is displayed:
CONFIG_TEXT: HTTP Error 503. The service is unavailable
In Event Viewer > Windows Logs > Application the following error:
CONFIG_TEXT: The Module DLL C:\\Program Files (x86)\\iisnode\\iisnode.dll failed to load. The data is the error.
PleskControlPanel Application pool is stopped at IIS Manager > server > Application Pools

Plesk users psaadm and psacln do not have access to C:\\Program Files (x86)\\iisnode\\ directory by default.

Apply the first solution below. If it does not help, apply the second one:

Repairing the disk security

Connect to the server via RDP.
Add the following lines into the disksecurity.xml file which is stored in %plesk_dir%\\etc\\DiskSecurity\\ directory:

CONFIG_TEXT: 
<Entry AccounType="1" Account="Psaadm" Path="{ProgramFiles}" SubPath="iisnode" AceFlags="ThisFolderSubfoldersAndFiles" AccessMask="Read" EntryFlags="0x40" />
<Entry AccounType="1" Account="Psacln" Path="{ProgramFiles}" SubPath="iisnode" AceFlags="ThisFolderSubfoldersAndFiles" AccessMask="Read" EntryFlags="0x40" />
Open Plesk Reconfigurator by running the following command in cmd:

C:\> "%plesk_dir%admin\bin\reconfigurator.exe"
Correct disk permissions:

OR
Repair Plesk Installation > tick checkbox Plesk File Security > Check.
Start Plesk Services Monitor:
Restart Plesk Management Service
Start PleskControlPanel application pool at IIS > server > Application Pools > PleskControlPanel > Start

The module removal

Back up IIS configuration file by running the following command in cmd:

C:\> copy "%windir%\\System32\\inetsrv\\config\\applicationHost.config" "%windir%\\System32\\inetsrv\\config\\applicationHost.config.bak"
Remove iisnode module under **IIS > Sites > PleskControlPanel > Module: