I don't see anything in their status page Zendesk Status about any services being down that might effect that. I would recommend opening up a support ticket with Zendesk with that error message and some details as to when it occurred: Sell – Zendesk help
We’re currently working through a service disruption and appreciate your report - it will help us better understand the scope and impact of the incident. Our team is actively working to resolve this issue, and we will provide more information as our investigation continues. Apologies for the inconvenience!
And then this:
Dec 14, 2023, 1:53 PM CST
After restarting affected services, latency has dropped and incoming and outbound email in Sell is processing as expected. We will extend our monitoring through to tomorrow to allow the team to investigate further and ensure complete recovery.