A fully managed end-to-end service for digitally signing code, documents, and applications. (formerly Trusted Signing)
Update: this issue is now resolved.
Although the verification link initially reported that it had already been used—and later returned a 504 Gateway Timeout—the identity-validation request eventually completed successfully without Microsoft reissuing the link.
The Azure portal now shows the Artifact Signing account as complete, the certificate profile is Active, and we have successfully signed and verified our Windows executables.
No further action is required on this request. It appears the validation completed asynchronously after the errors were displayed.