Hell's teeth. So today (January 20th, 2015) the DOI HTTP resolver at dx.doi.org started to fail intermittently around the world. The doi.org domain is managed by CNRI on behalf of the International DOI Foundation. This means that the problem affected all DOI registration agencies including Crossref, DataCite, mEDRA etc.
Geoffrey Bilder (Wed,) studied this question.
Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context: