It is impossible to challenge a friend to a game of brawl. To reproduce the bug:
1. Challenge a friend to a game and choose Brawl. Use the 2 crossed swords button to proceed, as otherwise the normal "challenge" button does not appear.
2. See that all of your legal decks are flagged as having one or more illegal cards.
3. Edit one of those decks and see that Arena is trying to checking to see if your brawl decks are legal for a 40 card format.
4. You can go no further
It is impossible to challenge a friend to a game of brawl. To reproduce the bug:
1. Challenge a friend to a game and choose Brawl. Use the 2 crossed swords button to proceed, as otherwise the normal "challenge" button does not appear.
2. See that all of your legal decks are flagged as having one or more illegal cards.
3. Edit one of those decks and see that Arena is trying to checking to see if your brawl decks are legal for a 40 card format.
4. You can go no further