Thanks for the fast reply.
This was the first thing i thought off and so i tried the same as you described but that doesn't help.
There must be another reason for this failure to occur (it's not server related - tried different ones).
Edit: I also tried to force "isConnected = true" even when (this._socket.Available == 0) -> = true.
And the download went perfectly fine. So (this._socket.Available == 0) reports a wrong value. Maybe a bug?
I also found this way (which you actually use) to truly determine if the socket is still connected as example in different forums. Can't imagine that i'am the first one who encounters a problem with this kind of test. Confusing :-).