Pop-up Email Forms have AJAX Error

I use an email pop-up sign-up form plugin called Optinmonster. It features some very well designed sign up forms. Unfortuantely, it has some bugs, one of which prevents visitors from inputting their emails. When these visitors click the “sign up” button on the form, an error message pops up saying “Ajax error” and nothing happens.

I’ve reached out to Optinmonster, and they aren’t sure what to do; I’ve had friends test this on their computers and it happens; it’s clearly a serious issue. I am not a professional developer so I can’t jump into the code editor and fix this without help.

Can anyone help? Thanks!