@ritchie same issue here after updating all my packages.

Getting this:

Oops! Something went wrong! :(

ESLint: 9.39.2

A config object is using the "extends" key, which is not supported in flat config system.

Instead of "extends", you can include config objects that you'd like to extend from directly in the flat config array.

If you're using "extends" in your config file, please see the following:
https://eslint.org/docs/latest/use/configure/migration-guide#predefined-and-shareable-configs

If you're not using "extends" directly (it may be coming from a plugin), please see the following:
https://eslint.org/docs/latest/use/configure/migration-guide#using-eslintrc-configs-in-flat-config

But don't really know how to fix, so I've just removed the extends bit from the config file.

Quick Housekeeping