State/Province auto-fill broken since 4.6.10/WP update

The State/Province dropdown auto-fill is broken in any form that it appears in, including in Admin=>Localizations=>'Languages, Currencies, Locations'. When I click on the dropdown it displays "N/A" and below it "None found." - selecting/deselecting other countries in the Country dropdown has no effect.

I recently upgraded to 4.6.10 from 4.6.3 - perhaps its related. WordPress version is 4.3.1.

I checked the 'civicrm_state_province' table in the db and the data seems intact. I’m not seeing any errors in the Console when I select a Country or click the State/Province dropdown with Firebug turned on, but I can see that the JSON object associate with the associated GET is empty (screenshot: http://imgur.com/Boq7xVP).

any ideas?