Do you live in a very humid area? Sometimes residual moisture can build up, and if the damage has already been done, that's why the message hasn't gone away. You can try powering the device down, giving a nice wipe inside with a Q-tip just to see if it removes anything, then take some rubbing alcohol and dip the Q-tip in it and try another rubbing. It could end up cleaning up some build upon those contact points. Let it dry after you do that, turn the phone back on and let me know if it starts charging.