Since the 1.02.00.00 MTG Arena update, players are experiencing an issue when trying to play with certain decks, getting the “Network Error: Deck validation failed.” message. This only occurs when the deck uses the Mirage basic lands (rewards from the Dominaria Plane-cation Chronicles – Standard Shakeup Event).
As a temporary workaround until the issue is resolved, removing these lands from your deck will fix the issue. For reference, these are what the impacted lands look like and their MTG Arena import codes:
The MTG Arena Status Page has the latest update for this issue, though there is no ETA at this time. Once we know more or when it is resolved, we will notify everybody so we can start using these awesome lands in our decks again!