Admin ajax not loading – Form Error

Hello guys,

I am using your plugin on the website http://speakreadwritethai.com/

I have never had a problem with this plugin but for some reason on this website the form has an error which outputs the following:

Oops! There is a problem with this form:
textStatus:error
errorThrown:Not Found
responseText:

And then displays my 404 page in place of the form.

When I inspect the console I get this message:

Failed to load resource: the server http://speakreadwritethai.com/wp-admin/admin-ajax.php?callback=jQuery111105487557002343237_1442639927623 Failed to load resource: the server responded with a status of 404 (Not Found)

My client is using namecheap server. I wonder if that is somehow causing the conflict? I have used this on other sites and never seen this problem.

Can you please let me know what’s going wrong?

Thanks

https://ww.wp.xz.cn/plugins/wysija-newsletters/