The built-in pull-ups are spec'd as between 20k and 50k, the relay is alleged to be 320 ohms, there's no way that makes a potential divider of 40%!
If the load on the pin is really 320 ohms then the chip is fried since it would produce about 4.5V into that load.
A good measurement to make would be the current into the relay winding when directly connected to 5V supply. If it reads about 15mA then it is 320 ohms and the output pin is blown (assuming no other load on the pin).