When sending a GET request (app version 1.4.15), the app reports:
{
"message": "Variables are invalid JSON.",
"name": "BadRequestError",
"stack": "BadRequestError: Variables are invalid JSON.\n at parseGraphQLParams (/Users/feri/Projects/IOW/IOW/app/node_modules/express-graphql/dist/index.js:254:38)\n at /Users/feri/Projects/IOW/IOW/app/node_modules/express-graphql/dist/index.js:234:12\n at process._tickCallback (internal/process/next_tick.js:103:7)"
}

For query variables:
{
"vatId": "12345678"
}

Sending a POST request works as expected. The GET request also works in version 0.4.2.