Some of our users have found that instead of inserting a pre-selected citation in their library into Word as per usual, the search interface opens with miscellaneous text appearing in the search box. Only happens with the first ref, all the rest insert correctly.
Rather than “insert citation” I think that the “find citation” button is being selected. Find citation, if no text is selected (in word’s toolbar) will bring up that window with the last search that was done.
Are you opening Endnote from the word Icon, or open Endnote first? Maybe they need to “find” each other first? Try heading over to Endnote from Word and see if it “fixes” the glitch?