Why Do I Get a 404 Page When My Popup Opens or the Submit Button is Clicked?
Improper /popup/my-popup Links Use
Make sure you're not using these links to trigger your popups. You will see these links when editing your popup, and they are for admin preview purposes only. When you are not logged in as an admin when using these links, you will get a 404 error on triggering your popups.
To learn about triggering your popups properly, check our Triggers Doc.
Email and Form Services Plugins Compatibility
You can also get 404 errors when you're using a form plugin that is not supported, or, your submit button does not support AJAX submission.
Here are two support threads for reference if you're interested:
Use a fully integrated and compatible form plugin such as Contact Form 7 or Ninja Forms. If you're in need of email only type opt-in forms, go with MailChimp or AWeber, which also gives you opportunity to use our MailChimp or AWeber extensions. All of these plugins are great options and will save you large chunks of time!