There is a Reset Jumper on the Melzi controller board that causes the board to reset every time the serial port (via USB) is opened. This can sometimes cause problems with the host software when it tries to connect to the printer. Removing this jumper can sometimes fix the problem. This involves opening the controller box and locating and removing the jumper shunt from the board. See the attached photo for a reference where the jumper is located.

Melzi Reset Jumper