REST API Playground gives only 412 "Precondition failed"

[bomas13]

Using the build-in playground ends for me in any case with a response like:

{
  "error": "Not allowed"
}

and HTTP status code 412 "Precondition failed".

Even for the Lokalize endpoint which is currently usable even w/o any token it's exactly like that.

However, using the generated curl command works just like a charm. Unfortunately, there is no verbose output for debugging - what can I do about this?