Update: It has something to do with a specific deck, I don’t know what tho.
If I use this deck list, I have the described problem:
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I created a new deck, with different cards, and had no problem siding cards or opening the deck editor multiple times. I also tested adding the cards that were different from the working deck and the “broken” deck one by one and tested if adding a single card made a difference. It didn’t.
I then cloned the working deck and restructured the deck list to be the same as the “broken” deck. That resulted in the same error. Changing the deck icon or name did not result differently.
It seems to me as if having those cards together in one list just breaks the game. Which is weird because it can hardly break the board of my opponents 