no answer when pushing the submit button (only ajax-loader)

Hello,

When a customer pushes the sent button the ajax-loader stays visible instead of an answer that the form is sent correctly(it is sent correctly by the way). I’ve deactivated all the plugins and activated them one by one. When I activate Nextgen Gallery (last version) the problem appears. Is there a simple solution for this or should I contact Nextgen for this problem.

https://wordpress.org/plugins/contact-form-7/