I looked and looked too, but did not find any visible signs of a solder break. I went ahead and rewet all of the joints anyway and it did indeed work. What is wrong with these boards is called a cold solder joint. The pins from the components and the holes and pads they are supposed to connect to were not heated to a sufficeint temperature to make the solder stick. Over time, the pins, holes and pads all develop corrosion which creates an insulator. By rewetting (melting the solder) the connections, you can make a better connection. If you add a little bit of solder as you rewet them, the flux (acid) that is in the solder's core (get flux core solder) will help eliminate the corrosion. Once done, it should last the life of the components on the board.
That cover was hard to get off. The best way I found was to insert a flat screwdriver into the slots and pry back the tabs. If you pry them back far enough, they won't bend back into place very fast. Go around quickly and it will come right apart. The tabs will regain their shape and make a solid lock again when you are ready for them to.