Worker Issue from default to custom domain

For Workes & Pages, what is the name of the domain?

https://gcb-dynamic-seal.tech-119.workers.dev/

What is the issue or error you’re encountering

I am experiencing an issue with the workers while uploading logs to my Elasticsearch engine.

What steps have you taken to resolve the issue?

Hi can anyone help with an issue I am getting with workers

I am experiencing an issue with the workers while uploading logs to my Elasticsearch engine. I am encountering the following:

When using the URL
https://gcb-dynamic-seal.tech-119.workers.dev/cd05db77-5f07-4359-7f92-48f98da85b00 I am able to send logs successfully.

However, with the custom domain
https://seal.cgcb.info/cd05db77-5f07-4359-7f92-48f98da85b00 I receive the following error:

"message": [
    "Failed to send log to Elasticsearch:",
    "error code: 521"

Could you please assist me in understanding why this is happening?

Thank you in advance for your help.

If you sure that 521 is returning by the Cloudflare, then you can refer to the 521 error documentation.