how did you ensure that the authentication was successful before. The only reason which i can think of is that the credentials which u are using for the authentication are incorrect or their might be some proxy in front your cluster. Kindly check the two.
If you are using 2 factor authentication, use access token instead of password.
Edit: make sure that your serviceaccount that is used to build image contains dockercfg credentials that you set when setting mendix operator. If you set it to default service account, add that credentials to the builder serviceaccount as well.