QgsNetworkReplyContent.content() empty after HTTP error

Question

I have a custom Flask backend and a QGIS plugin as client. I make a request with the QgsNetworkAccessmanager with blockingPost() to an endpoint which returns a JSON response on either HTTP status.
That works fine for HTTP 200, but when an API error occurs, the content() of the QgsNetworkReplyContent is just empty (b''). I'm fairly sure I'm missing smth on the Flask side, because in other plugins requesting other (non-Flask) APIs I can correctly see the content error message.. I tried diving into the source code to see what the Flask response is missing, but it's quite hard since the whole network module mixes Qt and Qgs components quite heavily.
EDIT: This is the (quite standard) error handler in Flask:
from werkzeug.exceptions import BadRequest

@api.errorhandler(BadRequest)
def handle_user_bad_post_error(e: BadRequest):
    return {'error': e.description}, 400

EDIT2: forgot to say that all other requesting methods I tried (curl, Postman) did response with the correct error message

ThomasG77 · Answer

If you are able to catch the status of the HTTP code answer e.g different from 200, you can't get the error message from method errorString and/or error from your QgsNetworkReplyContent object like illustrated below ?
I would try to change the URL from 2nd example with your own to see content of response.error() and response.errorString()
Working URL, HTTP 200
workingUrl = QUrl('http://example.com')

request = QNetworkRequest(workingUrl)
networkAccessManager = QgsNetworkAccessManager()
response = networkAccessManager.blockingGet(request)
status_code = response.attribute(QNetworkRequest.HttpStatusCodeAttribute)
if status_code == 200:
    print(response.content())
else:
    print(response.error())
    print(response.errorString())

"Unworking URL" e.g HTTP different than 200
unworkingUrl = QUrl('https://gis.stackexchange.com/error')

request = QNetworkRequest(unworkingUrl)
networkAccessManager = QgsNetworkAccessManager()
response = networkAccessManager.blockingGet(request)
status_code = response.attribute(QNetworkRequest.HttpStatusCodeAttribute)
if status_code == 200:
    print(response.content())
else:
    print(response.error())
    print(response.errorString())

QgsNetworkReplyContent.content() empty after HTTP error

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