I thought you would ensure that it worked before adding it.
And if I recall correctly, I merely reused one of the roots found in one of your server blocks.
I did not make that path up.
Had I, I would have asked you to create a new dedicated path for only challenge requests.
Review your code and the posts above.
In review, I think we may have gotten that "default code" from @_az
Who probably should have mentioned that you needed to validate it or adjust it to fit your needs - it was only being shown as a demonstrative example.
So...
The fix is to create that path or change all occurrences to an actual path on your system.