For future reference: putting an OK in the body appears to have fixed the issue.
Note that it needs to be OK with a new-line, as opposed to the subscription response where the challenge must be sent without a new-line.
For future reference: putting an OK in the body appears to have fixed the issue.
Note that it needs to be OK with a new-line, as opposed to the subscription response where the challenge must be sent without a new-line.