Google App Engine deployment failure: "The request is invalid for an unspecified reason." How to fix? - google-app-engine

Question

Our deployments for the default module of a particular app are failing, seemingly at random, at least 50% of the time, which is constantly disrupting our workflow.
With verbose logging turned on via appcfg.py update app.yaml --verbose, this error appears on the terminal:
03:43 PM Uploaded 4 files and blobs.
03:43 PM Compilation starting.
2015-09-23 15:43:51,886 INFO appcfg.py:1735 Send: /api/appversion/precompile, params={'version': 'myversion', 'app_id': 'myappid', 'module': 'default'}
03:43 PM Compilation completed.
03:43 PM Starting deployment.
2015-09-23 15:43:54,215 INFO appcfg.py:1735 Send: /api/appversion/deploy, params={'version': 'myversion', 'app_id': 'myappid', 'module': 'default'}
2015-09-23 15:43:56,341 INFO appcfg.py:2601 HTTP Error (HTTP Error 400: Bad Request Unexpected HTTP status 400)
03:43 PM Rolling back the update.
2015-09-23 15:43:56,341 INFO appcfg.py:1735 Send: /api/appversion/rollback, params={'version': 'myversion', 'app_id': 'myappid', 'module': 'default'}
Error 400: --- begin server output ---
Client Error (400)
The request is invalid for an unspecified reason.
--- end server output ---
The failure is extremely consistent, in that most of the time we try deploying the module after not having deployed for a few hours, the deployment attempt will fail with the above output.
Then, without changing any app code, retrying the deployment usually succeeds (but at times, the second attempt also fails, requiring subsequent deployment attempts).
This problem started happening earlier this year. Once the problem started occurring, it has not ceased. Before it occurred, we had no no issues with deployments.
The version of the module being deployed has no effect on the rate of deployment success. We are using the Python runtime for this module.
I have already emailed a Solutions Architect from Google about this, but apart from upgrading to a paid support plan to ensure someone looks into this, he suggested I post on here with the hope that the App Engine support team gets back to me.
App Engine support team - can you find out what is going on (and ideally provide a fix)? If you need more information (such as my app ID), please let me know.