Arduino UNO R3 + PCF8574P + HD44780

It still does not work.

You probably want to better inform your readers what "it" you were referring to. Having the latest code + schematic would be helpful.

It looks like your lcd is not initialized properly. I would focus on that.

As to the i2c address, given that it is 7-bit, different systems have a different way of representing it. Some would for example write the address as 0x40 (to simply OR or AND the i2c_read / i2c_write bit). Others use 0x20 and then left shift 1 bit first. You may want to read your lcd library to see which notation they use.