I am having problems with the same Ywrobot card on the back of a 20X4 display. Mine was sold by SainSmart but the provided code is marked Ywrobot. All I see is garbage characters for 2 lines.
Your problem is because the library provided by Ywrobot is for pre-1.0 Arduino. I have tried both the "official" I2C library and F Malpartida's NewLCD I2C library. I cannot display anything but garbage.