Have you found out what was causing the problem?

1) Can you check that the role you’re using for the web service calls has the correct permissions on the endpoint you’re trying to validate?
2) Does the role in question have the necessary access rights for that particular endpoint?​​​​​​
3) You should be able to check system logs to see if there are any additional details.
4) Does an administrator role work, or are you facing this error with any account?

Thanks

I only receive the same error message but already when I publish a certain customization. Even with the admin account. But it doesn’t say what is causing the problem. Since this was the only result that Google provided on this message I replied here.

I found out, that a certain feature was not activated yet. After activating it, the error doesn’t show up anymore.

And why on earth don’t you tell us what specific feature was not activated?