sometime if you try use the admin account of your facebook api 
it will show errors only to the api admin
if that shows no errprs 
then try login to reproduce the error then look at error logs may give you pointers as to whats going wrong

also a 3rd party plugin could be conflict with it try disable them and see if error stops

hello thanks for you help.

1/ You whant to sy i must lokk in my facebook developper account ?
2/Where can i find the error logs file please ?

:) thank you for your help because I just starting nox and this is a big trouble for me her/

http://club.wagen-connection.fr

Here the is my plugin list I have the minimum

Activity notifications Real-time and email notifications about site activity
Blogs User blogs with archives, tags, comments and rates
Cache Extreme Clear the cache while not in DEV-mode
Contact Us "Contact us" page with the ability to choose departments (email addresses)
Contact Importer Import Facebook, Google contacts
Events Create public and private events within your community, let people RSVP and discuss
Forum Simple discussion boards for users
Facebook Connect Allow users to join, sign in, and synchronize profile info using their Facebook accounts
Friends Friending functionality to be used across multiple features
Google Maps Location This plugin allows users to add their location in profile details. Plugin compatible with oxwall 1.7.3 and upper
Groups Simple groups within one site
Google Analytics Track your site visitor statistics with Google Analytics
Image Slideshow Image Slideshow
Messages Unified messaging plugin for private on-site communication
Newsfeed Newsfeed with likes and comments, Facebook-style
Photo Allow users to upload photos with tags, rates, and comments
Video Allow users to embed videos with comments, rates, and tags

and here is my API erro Tab and there is no errors :(

but just seven "Call" registered and more than 7 people try to use it today I think 30 or 40 people try it today.

and here are my app settings. Are thez correct ?

Ok Tammy thankk you I will try to make a test facebook account to see exactly.
Thank you

And after is in my facebook app i must look or in Oxwall to show errors ?

show advanced tab 

Here it is you think something is wrong ?

Yes,

My site is on : club.wagen-connection.fr

What must I put exactly for url here ?

ok I will try this. thank you

I have connected to your site via FB , it is working.

try login from devices enabled (advanced)

ok I have activate this :)

I will wait any users feedback now.....thank you for your help