在android移动后端启用身份验证时出现GoogleAuthIOException

我正在学习google app engine的移动后端入门教程（）。它可以完美地实现推送通知。然而，最后一部分，启用身份验证，让我夜不能寐。我一字不差地遵循了这一点，但我发现了以下错误：

02-01 00:11:08.497: I/CloudBackend(1982): error: 
02-01 00:11:08.497: I/CloudBackend(1982): com.google.api.client.googleapis.extensions.android.gms.auth.GoogleAuthIOException
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.api.client.googleapis.extensions.android.gms.auth.GoogleAccountCredential$RequestHandler.intercept(GoogleAccountCredential.java:308)
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:854)
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:410)
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:343)
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.execute(AbstractGoogleClientRequest.java:460)
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.cloud.backend.core.CloudBackend.list(CloudBackend.java:314)
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.cloud.backend.core.CloudBackendAsync.access$8(CloudBackendAsync.java:1)
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.cloud.backend.core.CloudBackendAsync$9.callBackend(CloudBackendAsync.java:270)
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.cloud.backend.core.CloudBackendAsync$9.callBackend(CloudBackendAsync.java:1)
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.cloud.backend.core.CloudBackendAsync$BackendCaller.run(CloudBackendAsync.java:402)
02-01 00:11:08.497: I/CloudBackend(1982): Caused by: com.google.android.gms.auth.GoogleAuthException: Unknown
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.android.gms.auth.GoogleAuthUtil.getToken(Unknown Source)
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.android.gms.auth.GoogleAuthUtil.getToken(Unknown Source)
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.api.client.googleapis.extensions.android.gms.auth.GoogleAccountCredential.getToken(GoogleAccountCredential.java:277)
02-01 00:11:08.497: I/CloudBackend(1982):   at com.google.api.client.googleapis.extensions.android.gms.auth.GoogleAccountCredential$RequestHandler.intercept(GoogleAccountCredential.java:301)
02-01 00:11:08.497: I/CloudBackend(1982):   ... 9 more

我曾尝试在云上重新部署，替换debug.keystore等，但同样的错误再次出现。 我也收到了这样的警告：

02-01 00:16:56.569: W/AbstractGoogleClient(3814): Application name is not set. Call Builder#setApplicationName.

此外，还有一个代码警告：

ExponentialBackOffPolicy类型已弃用

进口：

import com.google.api.client.http.ExponentialBackOffPolicy;

我不确定这些是否相关。任何帮助都将不胜感激。多谢各位