Error "custom keyword definition is invalid: data.errors should be boolean"

I thought that was something new coming with "@symfony/webpack-encore": "^0.23.0". However, there's an issue with the ajv library.

Dirty fix:

In

node_modules\ajv-errors\index.js

change the errors: parameter at line 14 from 'full' to true.

Clean fix (temporary):

Modify your package.json using this:

"resolutions": {
    "ajv": "6.8.1"
}

Reverting to a stable version of the ajv library also works:

npm uninstall ajv
npm install [email protected]