luceos I had the same issue. I deleted the unapproved entry in the flag table of the database and voila the Flag button works again. Thanks for pointing us in the right direction. I think it may be a minor bug related to a first post in a new discussion getting stuck in the approval queue and not letting the Flag posts load for some reasons unknown to me. Just posting here if it helps someone in the future needs help with the same issue. The javascript error which it throws is this for debugging if needed:
Uncaught TypeError: e.slug is not a function
at Function.t.route.discussion (routes.js:37)
at Function.t.route.post (routes.js:51)
at FlagList.js:34
at Array.map (<anonymous>)
at a.o.view (FlagList.js:29)
at a.e.render (Component.js:102)
at a.n.getMenu (NotificationsDropdown.js:47)
at a.n.view (Dropdown.js:42)
at a.e.render (Component.js:102)
at Object.o [as view] (Component.js:189)
routes.js:37 Uncaught TypeError: e.slug is not a function