The cause of this has been found due to a faulty NTP (timing sync) process on the central helpdesk server.
This has been restarted.
Any queued notifications may still be sent but these can be ignored, which may include notifications of an idle ticket (awaiting reply) or one that has timed out (closed ticket). These will not reflect the live ticket status.
We will continue to monitor the NTP process over the next 24 hours.