Whenever I have been playing Bo3 constructed (whether on the ladder or not), and I get into sideboarding, the error starts. I will be able to access my board and make changes, but after I submit 2 things happen.

1. The game gets stuck on the post-boarding screen and says that I'm waiting on my opponent for far beyond the allotted boarding time. I have to close and re-open the game, at which point I've lost the match.

2. I can access the board and make changes, submit them, wait for 2 minutes (every time) on the post-boarding screen, get into the next match, and the changes I've made have reverted back to the original, pre-sideboarded list.

See this thread for more details and others having the issue: https://feedback.wizards.com/forums/918667-mtg-arena-bugs/suggestions/40191766-sideboard-reverts-to-starting-deck

Sideboard bug is back. After disconnecting and reconnecting in a Bo3 game the sideboard becomes unusable. You need to wait for the timer to go all the way down until you can rejoin, but you can't make any sideboard changes.

Unable to sideboard in ranked bo3 -- when i start to sideboard i see a blank brawl interface with my companion as the commander. If I move my companion to my deck i can suddenly see my whole deck and sideboard, but cannot move cards from my sideboard to deck or vice versa -- when I move them, a copy appears in the place i am trying to move them, but the total number of cards increases by 1. Additionally, the player usernames are Username1 and LongUsername and the timer is stuck at 50 seconds. I am unable to click "done," but after the normal timer would end I am brought into game 2 or 3 with my main deck the exact same.

I experienced this same thing several times today.

As described in multiple posts, sideboarding with a companion is not possible in some matches.
The timer runs out, sideboard changes aren't taken into account, and cards that I should've had game 2 are still in the sideboard.
This has directly cost me a game where I had 4 hushbringers I couldn't bring in against an opponents ETB synergy deck after winning game 1.

As I lost the game on the ranked ladder to no reason other than a due to a supremely frustrating bug, the only option is to not to play arena until this is fixed.
Log attached.

I had the same issue in standard BO3. It had followed a first issue during the join of game 1 (blocked on VS screen). Could you go to game 2 (without sideboarding) or had you been kicked ? Regards.

bug bo3 cant side board

I experienced a bug several times after sideboarding with a Companion deck (Keruga). The new state of the deck won't be saved, cards I sideboarded out are still in and new cards are missing.

Steps to reproduce:
Build a deck with Keruga as a Companion and make a normal sideboard. (I can share my decklist if you want)
Play a game
Sideboard
See the cards that should be missing
Wonder why there are no sideboard cards

Expected Result:
Should be obvious, right? ;)

Screenshots: -

Workaround: -

Same problem here too.

when you get a disconnect in a BO3 match sideboard scree gets bugged, you cant see you deck nor sideboard, the timer its bugger also, if you restart the client get to that same bugged screen, mine says "Boros Angels" as deck name, sometime the "done" button works so i can get into the next game but no sb is a huge disadvantage, making me lose a lot of drafts and ranked games

Same. I was stuck in that interface during a traditional draft and lost a game because of it. Here's the log. Is a refund for the draft possible?

Happened to me when I lost connection to the server midgame. After the first game, we went to sbs and empty brawl deck.

skips sideboard screen in bo3 unranked play mode. happens even if game does not crash game 1. have run into this for the first time yesterday. can't find related complaints.

same here

Same for me... Twice already

Had the same issue

Had the same issue except I could still access the settings menu and could concede the game.

Hello,

For this issue, please submit at: https://mtgarena-support.wizards.com/hc/en-us/requests/new and select 'Report a Problem in your Event'.

Thanks!

This has been happening to me 2-3 times a day since the Historic patch was released.