Help Game crashes during online battle with friend

[Ytawo]

When battling a friend online, after about 10 turns the game crashes with this error message:


---------------------------
Pokemon Uranium
---------------------------
Exception: RuntimeError
Message: Script error within event 10, map 13 (Rochfale Town):
Exception: TypeError
Message: (eval):209:in `dump'no marshal_dump is defined for class Viewport
***Full script:
Kernel.pbTransferPlayer($game_map.map_id,15,5)
OnlineLobby.open

Interpreter:243:in `pbExecuteScript'
(eval):209:in `pbStartBattleCore'
(eval):191:in `loop'
(eval):275:in `pbStartBattleCore'
(eval):2554:in `pbStartBattle'
PokeBattle_BattleRecord:59:in `pbStartBattle'
(eval):33:in `start_battleonline'
(eval):32:in `pbSceneStandby'
(eval):34:in `start_battleonline'
(eval):31:in `pbBattleAnimation'

Interpreter:276:in `pbExecuteScript'
Interpreter:1600:in `command_355'
Interpreter:494:in `execute_command'
Interpreter:193:in `update'
Interpreter:106:in `loop'
Interpreter:198:in `update'
Interpreter:124:in `update'
Interpreter:106:in `loop'
Interpreter:198:in `update'
* Scene_Map:104:in `update'

We battled 3 times and this always happened to me at some point, never to him. Always the same stacktrace. Any idea? Let me know if you need more details.