The settings file for WeakAuras on that one character may have messed up, causing that error. Deleting that file would likely fix the problem, but will also reset WeakAuras on that character.
To delete that settings file, go here:
World of Warcract/WTF/Account/<account number>/<realm>/<character>/SavedVariables/
There, delete WeakAuras.lua and WeakAuras.lua.bak. Then log back into the game on that character.