Arena auto-passed when I had enough mana to activate ability.
I had Elvish Warmaster and several other elves in play, with 2 forests, Nykthos (5 green devotion), and Castle Garenbrig as my lands. I attacked with all my creatures intending to activate Elvish Warmaster's ability, which I had just enough mana for, but as soon as I had declared attackers, Arena auto-passed through the rest of my turn as if there was no more game actions I was able to take. It appears that Arena didn't recognise that I could use the mana from Nykthos to activate Castle Garenbrig's second tap ability, which would have given me a total of 7 mana that could be used to activate Elvish Warmaster.
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Ard_Rhys#31047 commented
I keep trying to attach the logfile, but for some reason each time I do it won't post the comment.