If you tried resolving a DOI in the past hour or so (00:30 to 01:30 UTC on 2024-05-29), you may have received a timeout notice on the doi.org domain:
While the issue now appears to be resolved, the International DOI Foundation reports on their status site that:
doi.org resolution services were inaccessible for approximately an hour from 00:30 to 01:30 UTC on 2025-05-29 [sic].
The issue seems to be caused by some failure of Cloudflare’s “Load Balancing” product which is used by doi.org to steer global traffic to appropriate backend servers. We have worked around this issue in order to restore doi.org resolution services, but have yet to restore full geo-steering functionality or determine why the failure happened. We will add information as it becomes available.
You can follow along here for more information. The International DOI Foundation’s status page is here and Crossref’s status page can be found here if you ever notice that something’s not working the way it should be.