i'm stuck on an alert mission and can't get out the "leave the group" button doesn't work
You have to leave the instance; leave group is trying to leave the teamup, which is not allowed on alert maps. Other than maps which trap you in combat (like Green Dragon), you can exit the instance by going out of combat and using Socrates as above. If all else fails, log out and wait (or log in an alt); once the instance you were in times out you'll log back in outside of it.
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Epic Stronghold
Block timing explained