"Betrayal at the Vault" was uncastable although nothing about the game state should have prevented me from casting it.
In the course of a game of OTJ quick draft, I had Betrayal at the Vault in my hand, a creature in play, legal targets on the battlefield, and sufficient mana available to cast the spell, but I was unable to cast the spell at any time.
More specifically, I had a Drover Grizzly on the battlefield equipped with Sword of Wealth and Power; my opponent had 3+ creatures in play (the problem lasted for more than one turn, until I lost the game); I had 10 mana available including 6 green mana.
I am playing Arena on PC.
Attached is the log of the game, which this bug made it impossible for me to win.
I will not be drafting on Arena again until this issue is resolved.
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DAC169#34537 commented
"I had a Drover Grizzly on the battlefield equipped with Sword of Wealth and Power."
Well, there's your problem. Your only creature had protection from Instants and therefore was not a legal target for Betrayal at the Vault.