I had a problem putting a new one in. Tried ones that used to work, but don't now. I guess it's something to do with uploading them that changed in the background as those, like yourself, who uploaded one before the change still work!
I think I fixed this recently - try uploading a GIF that's larger than 192x192px as your avatar, but less than 3MB. If that still doesn't work, try uploading the image you want to use to this post, and I will troubleshoot further
(For the nerds amongst us - PHP's in-built GD image processor couldn't handle animations, but could be fooled by uploading an image that was exactly 192x192px so it didn't process it. We switched to ImageMagick to prevent this, which does allow making thumbnails out of animated GIFs, but I think it got disabled again by an upgrade a while ago. I went in and reenabled it when I noticed.)