Ninja Form Getting stuck on processing

I am running 3 versions of this website; a Canadian, US, and European version. On Friday, the client called and said that one of the forms, which is used extensively across all the sites, was getting stuck while processing.

I made a change to the email alert action, which fixed the problem on all 3 sites. This morning, they have broken again. I ran the WP updates (none of which were related to ninja forms) and it is working again.

Is there a reason why the form stops working and then making a change to just about anything would get it to work properly again?

The page I need help with: [log in to see the link]