I am reporting a recurring MTG Arena client bug that prevented me from using the sideboard during a match.

The issue occurred on September 3, 2026, at approximately 8:44 PM CEST, while playing on macOS through Steam. I previously experienced and reported the same type of problem on August 29, 2026.

When the sideboarding phase began, the normal sideboard interface did not load correctly. Instead, unrelated deck-building, filter, cosmetic, pet, and profile interface elements appeared on the screen. The actual sideboard was inaccessible, so I could not make the necessary changes before the timer expired.

The attached log provides direct technical evidence of the failure. During the initialization of the sideboarding interface, it records the following call sequence:

SideboardInterface.InitializeDeckBuilder
SubmitDeckWorkflow.ApplyInteractionInternal

At that exact point, the client throws this exception:

ArgumentOutOfRangeException: Specified argument was out of the range of valid values.
No Pantry found that has registered type Core.Meta.UI.SceneUITransforms.

The associated stack trace includes CardBackSelectorPopup, DisplayItemSleeve, CosmeticSelectorController, PetCatalog, AvatarCatalog, and DeckDetailsPopup. This appears consistent with the unrelated cosmetic, pet, and profile elements displayed instead of the correct sideboarding interface.

The log also contains more than 17,000 repeated “ArgumentOutOfRangeException: startIndex must be less than length of string” errors involving CardTypeProvider.GetObjectTypeText and other card-interface components.

This was therefore a client-side interface failure, not a gameplay or connection issue. Because sideboarding is timed, I had no practical way to recover before the following game began, and the malfunction materially affected the match.