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I'll make this an answer rather than a comment so this question can serve as an anchor for this issue. The answer is the same as DavidPostill's answer to icons-missing-on-stack-exchange-sites. SU somehow got on the Fanboys Social Blocking List and ad blockers give various problems like that JavaScript message or elements of the page missing. The solution is to disable ad and tracker blocking add-ons for the SU web site (in my case, AdBlock Plus and Ghostery).