Zammad: Google login/auth not working, I get 422 every time

Same problem here btw. The callback URL is correct and the google account under the same email exists and owns the project actually where zammad got its client key + secret.. i tried to find some google logs for IAM / Auth but was not successful.

Is there any way to find logs for auth oauth requests to your project?

Thanks for posting.
I could verify this with a project that has never had authentication enabled - but:
This is not a Bug.

The cause of this Problem can be found within log/web-1.log:

I, [2018-11-16T10:39:42.637447 #23972]  INFO -- omniauth: (google_oauth2) Request phase initiated.
I, [2018-11-16T10:39:42.970970 #23972]  INFO -- omniauth: (google_oauth2) Callback phase initiated.
E, [2018-11-16T10:39:43.325612 #23972] ERROR -- omniauth: (google_oauth2) Authentication failure! invalid_credentials: OAuth2::Error, {"errors"=>[{"domain"=>"usageLimits", "reason"=>"accessNotConfigured", "message"=>"Access Not Configured. Google+ API has not been used in project [PROJECTID_GOOGLE] before or it is disabled. Enable it by visiting https://console.developers.google.com/apis/api/plus.googleapis.com/overview?project=[PROJECTID_GOOGLE] then retry. If you enabled this API recently, wait a few minutes for the action to propagate to our systems and retry.", "extendedHelp"=>"https://console.developers.google.com/apis/api/plus.googleapis.com/overview?project=[PROJECTID_GOOGLE]"}], "code"=>403, "message"=>"Access Not Configured. Google+ API has not been used in project [PROJECTID_GOOGLE] before or it is disabled. Enable it by visiting https://console.developers.google.com/apis/api/plus.googleapis.com/overview?project=[PROJECTID_GOOGLE] then retry. If you enabled this API recently, wait a few minutes for the action to propagate to our systems and retry."}:
{
 "error": {
  "errors": [
   {
    "domain": "usageLimits",
    "reason": "accessNotConfigured",
    "message": "Access Not Configured. Google+ API has not been used in project [PROJECTID_GOOGLE] before or it is disabled. Enable it by visiting https://console.developers.google.com/apis/api/plus.googleapis.com/overview?project=[PROJECTID_GOOGLE] then retry. If you enabled this API recently, wait a few minutes for the action to propagate to our systems and retry.",
    "extendedHelp": "https://console.developers.google.com/apis/api/plus.googleapis.com/overview?project=[PROJECTID_GOOGLE]"
   }
  ],
  "code": 403,
  "message": "Access Not Configured. Google+ API has not been used in project [PROJECTID_GOOGLE] before or it is disabled. Enable it by visiting https://console.developers.google.com/apis/api/plus.googleapis.com/overview?project=[PROJECTID_GOOGLE] then retry. If you enabled this API recently, wait a few minutes for the action to propagate to our systems and retry."
 }
}

You actually need to enable the Google+ API before being able to authenticate users via Google.
I'll be working on a documentation update, as this part of our docs is poorly written. Sorry for the inconveniences you may have encountered.

Visit the following link (you need to adjust it with your API-Project-ID) to enable Google+ API:
https://console.developers.google.com/apis/api/plus.googleapis.com/overview?project=[Your-API-Project-ID]

I'll be closing this issue - you can find the documentation issue here: https://github.com/zammad/zammad-admin-documentation/issues/10

Thanks, I can confirm that after enabling the Google+ API - now it works as expected.