It's heavily advised to only use one QNetworkAccessManager.
It doesn't look like you've encoded the query, maybe you could try that.
Inserts the pair key = value into the query string of the URL. The key/value pair is encoded before it is added to the query. The pair is converted into separate strings internally. The key and value is first encoded into UTF-8 and then delimited by the character returned by valueDelimiter(). Each key/value pair is delimited by the character returned by pairDelimiter(). Note: This method does not encode spaces (ASCII 0x20) as plus (+) signs, like HTML forms do. If you need that kind of encoding, you must encode the value yourself and use QUrl::addEncodedQueryItem.