Administrator can't insert page from template

Sorry, posted this problem yesterday, had it fixed this morning so I deleted the original post, but now find that the problem's returned!

Administrator can't add a page from template - when they click the green + icon the popup window appears for Content Name, but when they enter the name and click Add it just goes back to the main screen, no editable regions appear and no rows are added to the database. It works fine when logged in as Super Administrator, but it doesn't seem to be a permissions issue as an Administrator can open contents_insert.php directly. In fact, even if I temporarily remove protection from that page, it still doesn't work!