Unable to access Plesk after upgrade: Array and string offset access syntax with curly braces is no longer supported

Symptoms

• Unable to access Plesk GUI with the error:

  Array and string offset access syntax with curly braces is no longer supported (lessc.inc.php:717)

• Plesk was recently upgraded to 18.0.50 version.

• The Auto-update feature is disabled from Extensions>Updates.

Cause

The version of the Skins and Color Schemes extension is outdated.

Resolution

1. Log into the server via RDP.

2. Start a command prompt as an Administrator.

3. Execute the command below to update the Skins and Color Schemes extension:

   C:> plesk bin extension --install heavy-metal-skin