Okay I'll flag the question immediately, sorry for this!
Well I'm not sure for the plugin, because I deactivated them all and checked if Contact Form 7 was working, and yes it did!
Then I went back to my plugins and reactivated them one by one and always checked if there was any change.
I finished by having every plugin activated and Contact Form 7 working well too.
So if you have the same problem try deactivating ALL plugins exect Contact Form 7 and check if it now works. If yes you should be able to reactivate your plugins.