16x2 LCD shows only black boxes in the bottom line

OK, if we are looking at old topics, a little memo:
Do not connect the contrast potentiometer to Vcc.

If you have a board on which it is connected to Vcc, it will be beneficial (make contrast setting easier and reduce current consumption by 450 µA) to cut that connection (as long as of course, you do not cut any other connection to Vcc in doing so).