The final riddle was presented a few days later: "A hill Leo sees from a Birch TrEE", with a link to a GIF with a calculator on it. After some failed attempts at math, HondenMug (Dips) suggests typing it on a calculator and turning it upside down, like we did in the good old days with '80085'.

Walking to the coordinates gave nothing, so we tried swapping the x and z coordinate. Turns out it was correct! We solved the final riddle! The prize included some mapart, and clues to the 'Grand Prize' in the form of a book and a cryptically named nether star. However, I would have to work together with another player to get to this huge prize.