GAE/Poauth Invalid token signature occurs on a few percent of smartphones

Use Google Cloud Endpoint for Google App Engine/python servers
I am communicating with the Android app.

We are using OAuth authentication to identify users, and it will work well for the end of June this year
Yes, but among the 1,500 Android clients since the beginning of July,
Communication starts to fail at about 1-3%, and if you look at the GAE log, OAuth user authentication is
Failed

The error is mainly

id_token verification failed:Invalid token signature

However, sometimes

Signature verification error: (forgotten); continue with the next cert.

Errors similar to those shown in may also be mixed.

No changes have been made to the server or app since about a month before the problem occurred. There seems to be no law in the manufacturer or Android version.
Once a smartphone occurs, it always occurs, and reinstalling the app does not improve it.

Also, it may not be directly related, but the console
App Engine > Quotas > VIEW USAGE HISTORY
After the amount of money doubled to 6/29, 6/30, 7/1 was recorded twice, and until today
The date is not displayed.
(If OAuth authentication is successful, all processing is successful and there should be a charge.)
I have operated it for the past six months, but it has been very stable and has not happened.

I tried to change the Endpoints and pycrypto version, but it didn't improve.

As a question,
1. Is there a solution to the OAuth related issue?
Or did anyone have a similar problem?

2. Quotas/Did anyone have a similar problem with the usage fee?

3. What are some ways to get help from Google?
If possible, it would be better if there is no cost.

I'm in a lot of trouble because I can't find a clue to the solution.
If you have any information, please let me know.