The error message you're encountering, "ERROR: Type of GraphQL\Language\AST\ScalarTypeDefinitionNode::$kind must not be defined," is related to the GraphQL module in Plesk. This error suggests that there might be a compatibility issue or a problem with the GraphQL module after updating from Plesk Onyx to Plesk Obsidian.

To resolve this issue and regain access to the Plesk GUI, you can try the following steps:

1. Check for Compatibility: Ensure that the version of Plesk Obsidian you have installed is compatible with your server's operating system and PHP version. Review the Plesk Obsidian system requirements to make sure your server meets them.

2. Check for Extension Conflicts: If you have any third-party extensions or plugins installed in Plesk, they might be causing conflicts. Disable or uninstall any extensions that are not essential, and then see if the GUI becomes accessible.

3. Check PHP Version: Ensure that you are using a compatible version of PHP with Plesk Obsidian. You can check the PHP version used by Plesk by running the following command in SSH:

```
plesk bin php_handler --list
```

Make sure that the PHP version selected is compatible with Plesk Obsidian. You can change the PHP version in Plesk if needed.

4. Check for Updates: Make sure that you have installed all available updates for Plesk Obsidian. Use the Plesk Update Manager to check for and install updates.

```
plesk installer update
```

5. Check Logs: Check the Plesk error logs for more detailed information about the issue. You can find the logs in the `/var/log/plesk` directory. Look for any specific error messages that might provide more insight into the problem.

6. Revert to Previous Version: If all else fails and you cannot resolve the issue, you may need to consider reverting to the previous version of Plesk (Onyx) and contacting Plesk support for assistance. Be sure to back up your data before attempting a downgrade.

7. Contact Plesk Support: If the issue persists and you cannot resolve it on your own, it's a good idea to contact Plesk support for professional assistance. They can help you diagnose and resolve the problem.

Keep in mind that updating server software, especially control panels like Plesk, can sometimes introduce compatibility issues or conflicts. It's essential to carefully review the documentation and support resources provided by Plesk and seek professional assistance if needed to ensure a smooth update process and minimize downtime.