Ensure your .mul or .uop files are correctly mapped. On 64-bit systems, the client sometimes searches the wrong registry path for the "Patch" directory.
Check (essential for saving configuration files). classic client 6.3.12 for 64 bits
Use third-party tools (like UO Steam or Razor) to unlock the frame rate, as 6.3.12 was originally capped at a low FPS. Ensure your