Woocommerce Multistep Checkout plugin conflicting errors

My WordPress website has been working well with WooCommerce, 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.
If the user clicks to choose a different checkout method (E.g from Bank Transfer to PayPal) the page either continually tries to load the payment option on the screen but never completes or it takes around 40 seconds to switch methods.
(This has been tested on multiple computers/devices on multiple Internet connections)

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

Second problem,
Refreshing the Cart page or clicking to remove or update an item shows no items in the cart, even though the total cart price is still show in the top right corner of the header, and if you click on this price then the correct cart is displayed showing the items.

Third problem,
Checkout page load speeds. Clicking any of the following buttons takes an excessive amount of time to load a page: Add to Cart button, Update Cart button, Proceed to Checkout button, Continue Shopping button and also Place Order button. These all load slowly.

Any ideas? Could you please advise?

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