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The answer above is a bit incomplete, and will be misleading for anyone who comes across the same problem.
My guess is that at some point you copied the admin’s base_site.html template into your project, from an older version of Mezzanine. You’ve then later upgraded to a newer version of Mezzanine, which refers to an upgraded version of chosen – you can see the commit from 3 months ago here where that occurred here: https://github.com/stephenmcd/mezzanine/commit/f4e33282eaac44ef8ebbadb9b0157d910c67973a
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If anyone experiences this issue, check your javascript console. In my case, for whatever reason, the Admin section was trying to load mezzanine/chosen/chosen-0.9.12.jquery.js which doesn’t exist. I edited blog/templates/admin/base_site.html and updated it to mezzanine/chosen/chosen.jquery.js and the Meta Data section became expandable/collapsible again.