QExtSerialPort does weired things!

[xtlc]

I am reading a String from the Arduino, which comes like this:

string_outData = 'a' + string_1 + '&' + string_2 + '&' + string_3 + 'E';
  char char_buf[40];
  string_outData.toCharArray(char_buf, 50);
  Serial.write(char_buf);

string_outData = 'a' + string_1 + '&' + string_2 + '&' + string_3 + 'E';
  char char_buf[40];
  string_outData.toCharArray(char_buf, 50);
  Serial.write(char_buf);

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In QT, I want to receive the chars and build a string:

QString incoming_string = QString::fromLatin1(port->readLine());
       ui->recv_text->moveCursor(QTextCursor::End);
       ui->recv_text->append(incoming_string);

QString incoming_string = QString::fromLatin1(port->readLine());
       ui->recv_text->moveCursor(QTextCursor::End);
       ui->recv_text->append(incoming_string);

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This works fine so far - but this is a QTextEdit and not a string. In the QTextEdit I do not get 1 Line with the whole string, but this:
XXX // 3 chars
XXXX // 4 chars. ... why?
XXXX
XXXX
XX

instead of:
XXXXXXXXXXXXXXXXXXXXXXXX

What I want to get is the whole content of what is char_buf at the Arduino in incoming_string in the QT5 GUI. But I am failing miserably so far. I want to seperate the incoming_string later so I am able to work with the three strings. Please, I am very much in need of input How can I get a perfectly fine string ?