Unable to create a backup via WP Toolkit extension or create ZIP archive in domain File Manager on Plesk server: Path is invalid

Symptoms

• Unable to create a backup via WP Toolkit extension on Plesk server:

  PLESK_ERROR: Failed to back up site
  Path is invalid

• The following error is shown on the attempt to create a ZIP archive for any file in Plesk > Domains > example.com > File Manager:

  PLESK_ERROR: Unable to create the archive: Path is invalid

Cause

Malware file named nul.XXXXXX0000.cer blocks the backup creation.

With Plesk debug mode enabled, the following errors are shown in the %plesk_dir%/admin/logs/php_error.log file:

DEBUG Starting: filemng sysuser file_exists C:Inetpubvhostsexample.comhttpdocsnul.IIBBYI5454.cer, stdin:
Error code: 0, stdout: <XmlError Error="Path is invalid." ErrorCode="path_is_invalid" ErrorDetails="The filename, directory name, or volume label syntax is incorrect. (Error code 123) at C:Inetpubvhostsexample.comhttpdocsnul.IIBBYI5454.cer: contains reserved device names that cannot be used as the name of a file.
 at (RunTime::File::validateFileName line 2042)"/>

Resolution

Connect to the server via RDP and remove the malware file shown on the error message.

Note: Scan websites against malware using available tools: How to scan a website hosted in Plesk against malware