I was having this same issue. As it turns out the authentication method (ClientLogin) I was using to deploy to appengine was deprecated (I was deploying using mvn gae:deploy). Google now wants us to use their new OAuth 2.0 authentication for deploying to appengine: https://cloud.google.com/appengine/docs/deprecations/clientlogin
To fix this issue I had to update my appengine maven plugin to use the latest version so I could deploy.
I added this to my pom.xml:
com.google.appengine
appengine-maven-plugin
1.9.34
Then I used this command to deploy (instead of mvn gae:deploy):
mvn appengine:update
For more info you can check out this info on how to deploy to appengine: https://cloud.google.com/appengine/docs/java/gettingstarted/uploading
