I installed the 64-bit viewer using the MSI installer and would get errors if I tried to launch it from a .vnc file.
I found that HKEY_CLASSES_ROOT\vnc_auto_file\shell\open\command contained this:
@="\"C:\\Program Files\\UltraVNC\\vncviewer.exe\" \"%1\""
While it should be:
@="\"C:\\Program Files\\UltraVNC\\vncviewer.exe\" /config \"%1\""