We are aware of the recent issue causing Dungeon Reward Claim error and we sincerely apologize for any inconvenience it has caused.
Our team is actively working on a solution and we expect to have a fix released in the near future. In the meantime, we have received reports from some players that switching leaders, reinstalling the game, or trying a different connection (wifi/mobile) may resolve the issue, although we cannot guarantee these workarounds.
We appreciate your understanding and patience.