Can’t sideboard in cards only out
Tried to sideboard in between matches and it won’t add cards only remove them. I had to wait the timer out and didn’t get to change anything

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Grinnertoo#15738 commented
Had this happen to me numerous times. Game has been crashing constantly after some of the recent updates. I've noticed that this seems to be triggered by a game crash for me. If it crashes and I restart it and get back into the match I can not add anything from my sideboard into the deck between matches. I can remove cards though. I did notice that if I already have one of the cards I am trying to add from the sideboard in the deck I can plus up the number of them.
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ajkgo#79296 commented
This happens to me as well since the BLB update, I'm on macOS. This is super annoying as it makes the trad limited formats unplayable for me.