Order completion issue in woocommerce

Hello,
I am having a challenge with order completion in my woocommece store. When you select the bank transfer option, the page returns after clicking the place order button, instead of showing the thank you page with order information. The admin however receives an order received email but the customer does not receive any email. In Firefox, I see Internal server error on the top of the page. This is not the same when selecting the pay on delivery option as the customer and admin receives order received and confirmation mails for this option.

Now two things happened before this started:
1. I used a coupon code and the system said the coupon code is invalid and has been removed from my order
2. I edited the order on hold and order completed email earlier, although I have removed the text added, the same thing happens.

Please I urgently need help to fix this issue as it is a live marketplace.
Thank you.

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