Unable to access File Manager for a certain domain or for all domains on a server: Permission denied

Applicable to:

Unable to access File Manager for a certain domain or for all domains on a server:

PLESK_ERROR: Internal error: Unable to find the directory /var/www/vhosts/example.com/: filemng failed: filemng: opendir failed: Permission denied System error 13: Permission deniedMessage Unable to find the directory /var/www/vhosts/example.com/: filemng failed: filemng: opendir failed: Permission denied System error 13: Permission deniedFile FileManagerController.phpLine 1054Type PleskUtilException
Also, it is not possible to connect to this domain using FTP:

CONFIG_TEXT: Unable to connect FTP :421 Service not available, remote server has closed.
Alternatively, it is not possible adding files to archive in Domains > example.com > File Manager > Add to Archive:

PLESK_ERROR: Unable to create the archive: filemng failed: filemng: /usr/bin/zip execution failed: zip warning: Permission denied zip warning: Permission denied zip warning: Permission denied zip warning: Permission denied zip warning: Permission denied zip warning: Permission denied.

Incorrect permissions on domain's folder.

Fixing issue in Plesk:

Log into Plesk;
Go to Extensions and install Repair Kit extension as it is described in this article: How to install Plesk extension? and open it;
Press on File System > Repair.

Fixing issue over SSH:

Connect to the server using SSH.
Run the following command to repair file permissions by Plesk:

# plesk repair fs example.com
Checking virtual hosts' file system
There are files or directories with suspicious permissions in the
root directory of the domain 'example.com' ..................... [WARNING]
Correct the permissions for the affected folder and its , setting the system user and Plesk service group:

# chown -R System_User.psaserv /var/www/vhosts/example.com/httpdocs/
# chown -R System_User.psaserv /var/www/vhosts/example.com/error_docs/
# chown -R System_User.psaserv /var/www/vhosts/example.com/web_users/
# chown -R System_User.root /var/www/vhosts/example.com/logs

where
System_User - system user (main user) of the subscription that should be the same as shown in Plesk > Domains > example.com
Check permissions again and OK message should be displayed:

# plesk repair fs -verbose example.com
Checking virtual hosts' file system ................................. [OK]
Error messages: 0; Warnings: 0; Errors resolved: 0