Issue with NetConf : ArgumentOutOfRangeException

When an rpc-reply message is received, this exception is thrown in NetconfSession.OnDataReceived event handler.
The cause is that the received data length is 1015 byte, and the decoded message string contains a magic number at the beginning as below:
#1585

This number is matched with a regex then is used to append this length of data to a stringbuilder, namely to _rpcreply object, like :

var match = Regex.Match(chunk.Substring(position), @"\n#(?<length>\d+)\n");
if (!match.Success)
{
  break;
}
var fractionLength = Convert.ToInt32(match.Groups["length"].Value);
_rpcReply.Append(chunk, position + match.Index + match.Length, fractionLength);

where the value of position is the number at the beginning of the message, that is, 1585 in this example.
It is obvious that the Append operation will fail as one can't append 1585 byte from an 1015 byte length string.

What I can't figure out is where this magic number is coming from, as RFC6241 apparently does not contain such - lets call it - segment length element ? So where is it coming from, can anyone tell ?

Start in NetconfSession.OnDataReceived, focusing on the regex that reads the #length marker and the _rpcReply append operation. Compare this framing with the relevant NETCONF specification and inspect how received chunks are split. Done means identifying the marker's source and resolving the out-of-range failure for the reported rpc-reply.