Hi Cheezus,
Since you have to disconnect the entire click wheel to replace the battery, my guess would be that the connector isn't fully seated so you're getting most of the buttons working except one. I'd open up the bottom of the case again and reseat that plug. Pop it off and check for bent pins, then it wouldn't hurt to clean both sides of the connector with 90%+ isopropyl alcohol and reassemble. You can test it before you put it back together to verify if it's fixed or not.