First of all, thank you for using our plugin. Can you please provide some sort of screenshot which shows 404 including log description? Thanks.
Here is the unbounce error:

Is your website installed on root or any sub-folder or sub-domain may be?
it’s in the root domain. using WPEngine as a host
Could you please provide screenshot like attached below? That will help us more to investigate about the issue based on the response received from the server. Thanks.
https://drive.google.com/open?id=19CtykZcL39MHdrVzke04DIL2wB8ZFkZY
Please go through the steps ‘View Leads’ -> select any recent lead having issue -> select response
from the right box.
It looks like htaccess issue. Can you please uninstall and re-install the plugin? Let me know how it goes then.
Feel free to get back to us in case of any further help you may require. Also please share your review when you’ve a minute. Thank you so much!