Error: registration impossible... Please contact the webmaster
I think you're doing something wrong. You don't disable all plugins properly or you don't change the theme properly. This is not wpForo error for sure. wpForo doesn't have such words in core, this is impossible. As I said this come from other places and you should find it. There is no chance to fix it from wpForo side.
Navigate to your website native user registration page and see that this error is not related to wpForo: http://www.apedys.webiaprod.com/wp-login.php?action=register
I did several tests and I saw that it was when I syncronized all users that this problem happened. I have imported into a new site all my without the database and everything work and as soon as I import my database the problem start over so I know it comes 100% from the database but now I don't know how to find where it comes from in my database.