This. Resetting flash points didn't work. Disbanding and regrouping didn't work. Abandoning all flash point quests (separately and in the group), resetting flash points and then re-taking the missions didn't work. Finally got it to work by using an old WoW hack If you remember how to reset instances in classic WoW, before they 'allowed' it, it's the same thing: Reset instances and get everyone in who can get in. The person who still can't enter needs to leave the group. That person then invites someone else to a new group. Promote the new person to leader and leave that group, then get re-invited to the old group. This should set the 'saved' flashpoint to the person who became 'new' leader, as flash points are saved to group leaders and sometimes that bugs out, apparently. You should now be able to enter with the old group.