addons-server: 504 error when trying to upload the source code of a new version

Describe the problem and steps to reproduce it:

(Please include as many details as possible.)

1. Go to AMO -dev: https://addons-dev.allizom.org/en-US/developers/addon/submit/upload-listed
2. Submit a new add-on (doesn’t matter which one as long as it passes validation, you can use a dummy one from https://staticfil.es/webext-generator/)
3. On the next page, check that you want to attach source code to the submission
4. Select a zip file that is rather big (e.g., I tried with a 50MB zip file)
5. Click to continue

What happened?

This error is displayed:

504 ERROR
The request could not be satisfied.

CloudFront attempted to establish a connection with the origin, but either the attempt failed or the origin closed the connection. We can't connect to the server for this app or website at this time. There might be too much traffic or a configuration error. Try again later, or contact the app or website owner.
If you provide content to customers through CloudFront, you can find steps to troubleshoot and help prevent this error by reviewing the CloudFront documentation.

Generated by cloudfront (CloudFront)
Request ID: 0hn0qJOZeCrgZLbhjaPj4mtquw6NDlZ4re9fB70kbvnGcLA8jMBUUw==

What did you expect to happen?

No error.

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