Error when submitting newsletter sign-up form

### Summary

Error when submitting the newsletter sign-up form on https://ubuntu.com/contact-us/newsletter-opt-in

### Steps to reproduce the behavior

1. be logged in on the site (I haven’t tested whether that is relevant)
2. go to https://ubuntu.com/contact-us/newsletter-opt-in
3. enter email address (I used the same email address that I also used for the account that I’m logged in with)
4. click the checkbox to agree to receive information
5. click “Submit form”

### Expected behavior

Either Proceed in the subscription process to the stage where I’d confirm the email address, or ideally — since I used the same email address that I also used for the account that I’m logged in with, which is already confirmed — immediately complete the subscription process.

### Browser/device details

PC
Debian 12.13
Firefox
140.10.1esr (64-bit)

### Reported from

https://ubuntu.com/contact-us/newsletter-opt-in

Start by reproducing the failure at https://ubuntu.com/contact-us/newsletter-opt-in using the listed Firefox and Debian environment, while checking the browser's visible error and developer-console output. Trace the form submission from that page and confirm that a valid email can proceed to confirmation or complete subscription without an error.

Written by the indexing model from the issue text.