We're still experiencing this issue today. Using your SDK, we've created and loaded an apiContext for psd2 (with the sandbox environment type) and created a new OAuthClient, which we use to generate the an OauthAuthorizationUri, all without any problems. But the page displays "Connection can't be found". Do you have a solution for this by any chance?
Creating the link manually and using the ClientID retrieved from the sandbox bunq app user generated by tinker does work.