AWS Cognito Integration

I’m developing a serverless application on AWS using Cognito for user management.
Evaluating for the FE but I just can’t get the bubble API plugin (oauth2 user-agent) to work with Cognito.

I can get Bubble to call the /token endpoint but I get back this 400 error

The error from Cognito does not give me any idea of what it is unhappy about.
Is there a way I can see what Bubble is calling Cognito with?

Another hint. When I select “Token is returned as querystring”, I can see that the /token endpoint is returning the access token (just not as a querystring). So I know that auth succeeded.

After 3 days, I’m out of ideas and might have to give up Bubble.
Has anyone gotten the oauth2 user-agent to work with Cognito?
Could you give me a hand?

BTW, here is one out of dozens of settings I’ve tried.