The serveur needs to contact google reCaptcha validation server in order to work. This requires calling an "https" address. The "https" protocole can not be used if the trustStore (e.g. tomcat argument "-Djavax.net.ssl.trustStore=" ) is empty. This problem must be solve on the server configuration.
There was the problem that we didn't have any log, the patch attached to this issue should fix that.
It was due to a bad tomcat configuration