q&a is not working with friendly url

I've tried to modify QA_SLUG_ROOT in qa.php and now, the plugin pages don't work, the404-errorpage appears. I've undone the modification and it still doesn't work.
I've tried to download the last version of the plugin and i've installed it but it's still not working. What could I do?
I've seen that if I disable friendly url in wordpress the plugin works but I need to have my have my friendly urls working.

I have the same plugin installed in an other domain with the modificated slug and it works well.

Thank you!!!