Hopefully this is the final update for this thread! I came back to this problem after working on a couple of other projects for a bit. After creating a fresh project and painstakingly copying the Content folders over one-by-one until the function broke again, I finally found the issue.
For some reason, it was caused by one of my sprite textures not being a power of two (I was actually aware of that, and was planning to fix it later on). As soon as I made the texture the correct size, the leaderboards started working again. I knew the texture would not show correctly on Android, but I had no idea it would cause such a horrendous conflict! Still, happy that it’s finally fixed 