["unable To Join Game. Returning To Menu"]

My brother and I play warframe and decided to play as a party. We joined a party together and played one mission perfectly fine. After that mission, it will not let us join each other's game. We are using the same router and our connection is the same (~15mbs down and ~3mbs up. NAT 2). We are both able to play with other people separately but are not allowed to join the same game. Any ideas on why/how to fix this?