Hi there, Thanks for purchasing the game!
I also have an Ultimate 64, and it works for me. I have however not updated the firmware in quite a while. Could you try running the U64 in Pal mode and see if that helps?
I have also heard that therehave been issues with certain cartridges when running on the U64. Try unplugging any cartridges if you are running the digital version of Rogue64.
Let me know how things go. If you can't get it to work I will try upgrading my U64 to reproduce it. 🙂