Fix backend API error and complete registration flow

I need a web developer or network specialist to debug a persistent backend error during the registration process on a specific e-commerce platform.

The Issue:
When submitting the standard registration form, the request hangs for ~6 seconds and returns a generic server error instead of completing.

Required Deliverables:
1. Analyze the HTTP requests, APIs, and network configuration to identify why the server is rejecting the connection.
2. Successfully resolve the block (must use a clean Spanish residential IP environment) and complete the registration flow.
3. Success criteria: Trigger the standard green confirmation message on the site asking to check the email for the activation link.
4. Deliver the test account credentials and handover the associated email used, plus a brief technical explanation of the fix.

Payment Terms:
* Fixed-price project. Payment released upon verification of the working login and email handover.