I re-uploaded the files that go with the mod that I didn't initially because I thought they'd all been saved from the old forum. After we had this glitch I had a hunch they weren't all there, so I overwrote the existing folder on the server with the one on my hdrive I knew had all the working files.
Low and behold, you can click on the smilies again and not get an error.
The buttons above the post box now work again too. Problem solved.