Monitoring - The service appears stable now, and we are processing requests. We are monitoring now with the changes we've made.
As mentioned in the previous update, do note requests for Crossref DOIs via DataCite Content Negotiation may still return a 404 and this is expected behaviour at present.
Jan 19, 18:56 UTC

Identified - A large number of requests has been identified that are specifically attempting to resolve Crossref DOIs via the DataCite content negotiation service, we are having on-going conversations about this use-case.

To remedy we've implemented some timeout logic and additional rate limiting, this however may have a knock on effect those attempting Content Negotiation for Crossref DOIs via DataCite Content Negotiation (Crossref Content Negotiation or via doi.org is unaffected) may receive a 404.

Requests for DataCite DOIs via Content Negotiation should now start to be resolving.
Jan 19, 15:06 UTC

Investigating - During the past week or so we've noticed a huge surge of increased traffic to the Content Negotiation Service. This has unfortunately caused various problems with requests not being resolved. This is currently affecting both the Content Negotiation service but also the Citation Formatter service.

We've made some initial improvements to support the increased load on the service, but it is unfortunately still causing issues.
We will continue to investigate.
Jan 11, 11:00 UTC