Plesk is inaccessible: 500 GraphQL: Unexpected Name "MainMenu"

Symptoms

Plesk is not accessible, with the following error:

Type GraphQL\Error\SyntaxError
Message Syntax Error: Unexpected Name "MainMenu"
File Parser.php
Line 433

Cause

There are corrupted or missing files inside %plesk_dir%\admin\cp\queries.

Resolution

Replace the corrupted/missing files with ones from a new installation:

1. Log in over RDP

2. Open a Command Prompt and check the Plesk version

   plesk -v
   Product version: Plesk Obsidian 18.0.68.1

3. Open the Plesk for Windows packages page in a web browser

4. Navigate to PANEL-WIN_<PLESK_VERSION>/ > dist-windows/

5. Download plesk-control-panel.zip and extract it

6. Rename %plesk_dir%\admin\cp\queries to something else with the File Explorer

7. Move the new queries folder to %plesk_dir%\admin\cp\queries

Note: These files were likely removed or corrupted by antivirus software. Make sure to exclude the Plesk installation in the antivirus software