I contacted the seller and he notify me that it is very strange and they have never saw the same issue.
The only thing that i cannot understand is that i had bought two lcd with the arduino Mega and both(!) had the same issue.
I guess you can rely on him to say that, and having two the same is certainly strange, but I suppose it is possible to have a batch with the same fault and it might ultimately be best to try for a refund.
I said the documents were confusingly packaged because the packaging is confusing. They are also incomplete. Needless to say, I have no idea if the library has been modified or not, but it costs nothing to try the download from http://www.henningkarlsen.com/electronics/library.php?id=51
thereby ensuring what you have is original, kosher and the latest version. This guarantees nothing and it didn't work for me, but it might for you and a lot easier than getting a refund. I recall there was only one 400x240 example but you get some other examples to try, which did work for me. As I said, there is only one library and the vital change is the model identification. FWIW, I use
for my 400x240 but the line came out of a 480x320 example.
What I don't understand is why you get what you get and nothing else. The demo goes through a raft of exercises and is even animated, but all you talk about is a square. You might add a few serial.prints as markers, to check that the the programme is running OK.