You might want to delete both of these files.  It fixed the problem for this user.

Tom suggests this one: “the .plist with CharacterPalette in its name in Home/Library/Preferences”

balto suggests this one: “If that doesn’t do the trick, also delete the com.apple.HIToolbox.XXXl one located in the ByHost subfolder, which popped up as updated when I checked the character viewer.”