WooCommerce Payment Processing Error Fix

I'm experiencing a payment processing error in my WooCommerce site. The issue arises during the checkout process. It's not linked to any specific payment gateway, as I'm using a custom one.

Ideal Skills:
- WooCommerce troubleshooting expertise
- Custom payment gateway integration knowledge
- PHP and WordPress proficiency

Please provide a detailed plan on how you will approach this issue and your estimated timeline for resolution.