Please update the MC4WP plugin to the latest version first. If that doesn’t solve the issue. please see https://www.mc4wp.com/kb/fix-the-oops-something-went-wrong-error/
If you can still not solve the error after reading that kb article, please let me know the error you see in the log under MC4WP > Other.
I have trouble loading your site / scrolling down, so I wasn’t able to test the form myself.. The form in the popup isn’t made with MC4WP.
Hope that helps. If you have any questions, please let me know!`