Sorry, my fault. I forgot to change one thing in the code. Look for these lines:
//LiquidCrystal_I2C lcd(0x3f, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE); // Set the LCD I2C address
LiquidCrystal_I2C lcd(0x20, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE); // When simulating in Proteus use this one
You need to uncomment the first line and comment the second one. It's somehow needed for my simulatio to work.
It should look like:
LiquidCrystal_I2C lcd(0x3f, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE); // Set the LCD I2C address
//LiquidCrystal_I2C lcd(0x20, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE); // When simulating in Proteus use this one