While essentially correct, the "problem" with your solution is that if you make the changes to the AdventureLeaderboardWnd in your default directory (as would be expected), the next time you log in, it will just re-download the existing one, overwriting the changes you have made.
The only way this will actually "work" (until SOE actually uploads the updated file) is for people to make a copy of the EQUI_AdventureLeaderboardWnd.xml file from the default folder and put it in their custom folder,
then apply the changes you have posted.
The problem there becomes the age-old problem of having files in your custom folder that aren't
actually custom, creating the opportunity for it to be "broken" in the future.
It's annoying to see the red error message on the screen, but
this error, in and of itself, will not prevent any UI from loading (tested with my own). My suggestion to folks would be to just put up with the annoying message (provided it is
only due to this one thing) until SOE uploads the proper file for the default folder. (And yes, they have been made aware of the fact that they overlooked this file.
)