this has happened to me twice now, I would que in duty finder with a premade party if one of my party members rejects the duty when it comes up, I get the 30min ban other player can rejoin the duty finder with no issues, both times I have accepted the duty, only to be banned.

details:
first time it was in a pre made party of 6 for turn 5 (nobody ban me got banned)
second time it was in a premade party of 2 for front-line

things that might effect it
I changed servers not to long ago from coeorl to Tonberry
I am running windows 8