For me it is any deck. No matter if I go the route of Decks button or from the Play screen, via changing a deck.

I had the same issue so I deleted the deck and remade it, soon as I started tinkering with the sideboard it bugged out again. Seems like the issue starts with the sideboard having weird glitches, then the deck becomes inaccessible if you close it and try to get back in. Idk what to do, I basically can't build a Bo3 deck currently.

I'm also having this problem. I get stuck on 'waiting for the server' if i try to modify my deck I just made. I think it's an issue with the sideboard, because I got a weird error when saving the deck about the sideboard being too full; but i only had put 14 cards in.

Similar Issue. This is a Standard Deck - Naya. This just started recently. I have opened and modified this deck within the last week or so.

I have the same issue with a mono white angel deck

Could it be due to the following problem?

In my particular case, I have a deck (which I can't edit) which is incomplete because I don't have all the cards yet. In this incomplete version, I have 2 lands in my case 2 pieces of Concealed Courtyard. When I built the deck I had used 2 versions of the Kaladesh version. But I only had one of them at the time. Now I pulled a second Concealed Courtyard from a booster. This time the Thunder Junction version.

So I have 2 Kaladesh versions in my deck, of which I only have one. Now my second copy, the one from Thunder Junction, is automatically added. The deck actually expects two Kaladesh versions, but now gets another second copy.

expected result: 2x kaladesh concealed courtyard
actual result: 1x kaladesh, 1x thunder junction concealed courtyard

Maybe that's the problem???

I got the same issue and it's very annoying.