Unfortunately, there are more DOIs that resolve to broken links than we would like. This is because the URL that the DOI link goes to needs to be updated by the publisher when that URL changes -- and not all publishers reliably update their Crossref records.

The best thing to do when you encounter a broken DOI is to report the issue to the publisher and keep the DOI in your reference. The DOI is correct, even if the link it points to is currently broken -- and since Crossref sends monthly "resolution reports" to member publishers (listing failed attempts to access their content via DOI links), a broken DOI link will help remind the publisher to update their records.