Woocommerce Multistep Checkout plugin conflicting errors

My WordPress website has been working well, but I recently installed Woocommerce Multistep Checkout plugin to neaten up the checkout process and now I am having troubles switching between payment methods (Bank Transfer, PayPal, & Omise) on the final checkout page.
This plugin has no support other than a FAQ which only has five answers, non of which are relevant.

If I click to choose a different checkout method (E.g from Bank Transfer to PayPal) the page continually tries to load the payment option on the screen but never completes.

Google Chrome inspector picks up the error:
Failed to load resource: net::ERR_CONNECTION_RESET
/?wc-ajax=update_order_review

Here is a screenshot of both the checkout & the error message:
https://ibb.co/iJ8ufw

Any ideas anyone?

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