gactions test fails with “Your app must have at le

Question

When I run gactions test for one of my Actions SDK projects I get the following message:

     ERROR: Failed to test the app for the Assistant
     ERROR: Request contains an invalid argument.
     Field Violations:
     #  Field                         Description
     1  agent_version_content.action  Your app must have at least one action for locale en
     2017/10/09 17:48:46 Server did not return HTTP 200

This has also been raised at least twice in the Google+ community - so far without any proper answer.

What can we do, to get rid of this problem? What is the cause of it?

If I create an empty project in the Console for Actions on Google and upload and test the project with this new project-id I do not get this message.

Another interesting hint: All information for this project (apart from it being there) in the Console was lost. I had to reenter all the stuff like app icon and so forth.

And all the time the Console pretends that I didn't upload anything. The first step of the five-step wizard has no check mark but simply says "1" and the button "Add Actions" is highlighted. but the gactions update returns successfully. For the newly create project, though, the step one has a checkmark.

Edit (Oct, 10th - 16:10 UTC): Now I get an error message when trying to use gactions update:

     ERROR: Failed to update the app for the Assistant
     ERROR: Precondition check failed.
     Violations:
     #  Description
     1  Draft already uses en as a language so it cannot be used again for localization.
     2017/10/10 18:11:55 Server did not return HTTP 200

This happens - no matter if I use action.en.json (including "locale": "en" in the file) or action.json as filename (and not including the locale information in the file).

Same result with gactions test.