Problem on "Ask a question"

Hello, when we click on "Ask a question" button show the page "Page not found". We found some probably solutions, but it doesn't work. What can we do? Our time is gone and we need it urgent.
Tries:
- disabled all plugins and doesn't work
- changed the theme and doesn't work
- saved the permanent link to default and back to customized and doesn't work
- tested in another browser and doesn't work.
Actually we tried everything there we found on forums and nothing.
Our address site is http://107.21.255.194/