[Resolved] The auto-renew of the SSL certificate on api.smsbroadcast.co.uk failed, causing the existing certificate to expire at midnight on 18th March. Applications and users accessing api.smsbroadcast.co.uk over HTTPS/SSL would have received a "certificate expired" notice. The certificate was manually renewed at 10:02am on 18/03/2019, and the auto-renew scripts rebuilt with better monitoring in place. SMS delivery and webhook callouts were unaffected.