Just had the same issue. Possibly triggered by a disconnect from the server during the game before? Able to move cards from my maindeck to my sideboard, but not the other way around. Playing on windows, not steam

Insanely tilting, just happened to me.
WTF!!!
In my case (PC), game froze during the first match of the bo3.
Had to restart the pc shutting it down it wont let me change windows or do anything.
When I got to enter the game again, I kept playing and won the first match.
Then I was unable to sideboard, cards wont move in either direction... Lost second match and happened again between second and 3rd....
Very very frustrating.

Your game client is almost unplayable after 6 years! Seriously, what the **** is wrong with your company.

Same thing happened to me, unable to sideboard before game 2, restarted client and still unable to sideboard. Between games 2 and 3 is the exact same thing, can take cards out, cannot bring cards in. And I believe, but am not certain that my opponent was able to sideboard as Phoenix decks don't usually run Mystical Dispute maindeck.

I ********* pc aswell, almost every game. super tilting

This is also true when playing arena on android Pixel 8pro.