This issue was caused by a change in the SSL/TLS certificate chain during a scheduled renewal.
GBG investigations have confirmed that there were no errors in the execution of this change, which had been properly tested in advance, and was deployed in line with industry best practice.
Analysis indicates that affected customers experienced issues primarily because their systems, especially those utilising legacy Java implementations, lacked the updated root certificate in their trust stores. This omission led to failed SSL handshakes and subsequent service disruptions.
The certificate was rolled back to restore service.
We highly recommend ensuring your trust store is kept up to date going forward.