Companion Sideboard Bug
Play with Gyruda, Doom of Depths as Companion
Add odd cost cards to the sideboard, which by the way, is legal
When you sideboard, it just doesn't let you and all the cards disapear
You are allowed to sideboard in odd cards, this bug should not happen.
Don't know workaround

same with a Yorion deck--during SB, does not show any cards in deck but Yorion, refuses to accept the "done" button...I haven't tried waiting until sideboarding time runs out, I've just ended up conceding twice now

In a draft event just now I encountered a bug when sideboarding with Keruga as my companion. Event is traditional draft. I did not encounter this bug all the time when sideboarding, only once. When Keruga was selected as companion I could not make any changes to my sideboard or submit my deck, and the interface did not show any cards and showed variable names for things like the deck size -- see attached screen shot. Removing Keruga as companion allowed me to make changes, but I was also not able to submit my changes while Keruga was my companion (clicking submit did nothing). I closed and restarted the client, which did not help. Eventually sideboarding timed out and I was able to play the match. I don't know if my changes were saved or not.

Yes seems like it's a problem related to BO3 decks with Yorion as a companion, I've found this bug too. Please upvote my feedback too, maybe in this way they can realize it faster:
https://feedback.wizards.com/forums/918667-mtg-arena-bugs/suggestions/40210294-lost-a-traditional-draft-game-because-i-couldn-t-s

After reconnecting to a game that was currently sideboarding, I encountered this screen and was unable to make any changes.

I'm experiencing the same issue as this user. Mine has happened in 100% of the games.

After my game one. I went to sideboarding and it didn't allow me to. Showed me this screen and wouldn't let me proceed. Had to concede to exit even after waiting a while for the "sideboard time" to finish.

I'm getting this same bug over and over when I play with companions. After game 1 ends, the client freezes up and I get a screen looking like this. Clicking on any buttons or cards does nothing, the only buttons I seem to be able to access is the ones in the settings menu. Cards aren't in play during game 2, and sometimes I just time out waiting for it to complete and see a defeated screen.
Steps to reproduce:
Play a Bo3 match with a companion.
Seems about a 50% chance that for game 2 this screen will appear
Results:
Screen as shown in attached photo. None of the sideboard cards are in my deck for the 2nd game, and often I time out
Expected results:
Should be able to sideboard and not time out and lose rank to this bug

Unable to sideboard on bo3 after disconnected.
So i restart my game after i got disconnected in the middle of the game, after first game done i cannot see my main deck, only sideboard card shown.

Attaching my log as I just had the same issue

Deck appears to disappear and the 'done button' doesn't work. My companion is also missing as you can see. Sideboarding is impossible and the only way to get to the next game of the match is to wait for the timer to run out (which also isn't visible as the onscreen timer is stuck at 50 seconds). The player names and scores also appear bugged.

I don't know if this is specific to decks with companions, but I've encountered this bug 3 times and each time I was playing a deck with Yorion as its companion.

I cannot see my deck, and I don't know what's happening. I click done and stay at the sideboarding screen, even after restarting client.

This just happened for me as well. Luckily it didn't overly effect the game but it does seem to be connected with using a companion .

it wouldn't let me edit my sideboard during best of 3 and showed a very glitched out screen for the duration of the sideboard time. See image added below.

Unable to sideboard in ranked B03. Since the update this morning 24/4/20 connection issues have been awful and I cannot sideboard in B03. All I see is my companion slot and the only option was for me to concede the match (lose my rank) and restart the client. Not good!

Same with Umori:

Companion
1 Umori, the Collector (IKO) 231

Deck
4 Gingerbrute (ELD) 219
4 Manifold Key (M20) 230
2 Shadowspear (THB) 236
4 Steel Overseer (M20) 239
3 Clockwork Servant (ELD) 216
4 Mystic Forge (M20) 233
2 Pattern Matcher (M20) 234
2 God-Pharaoh's Statue (WAR) 238
2 Meteor Golem (M20) 232
4 Stonecoil Serpent (ELD) 235
4 Castle Locthwain (ELD) 241
1 Blast Zone (WAR) 244
19 Swamp (ANA) 58
1 Wand of Vertebrae (GRN) 242
4 Inquisitive Puppet (ELD) 223

Sideboard
1 Umori, the Collector (IKO) 231
1 Grafdigger's Cage (M20) 227
2 Legion's End (M20) 106
2 Soul-Guide Lantern (THB) 237
2 Sorcerous Spyglass (XLN) 248
2 Duress (M20) 97
3 Agonizing Remorse (THB) 83
2 Epic Downfall (ELD) 85

Playing Traditional Standard Ranked with a (Keruga) Companion Deck.
During Sideboarding I cannot see the cards in starting deck.
Even names of Players are incorrect, saying somewhat like "UserName". Time during sideboarding does not flow and only thing I can do is wait for second/third game without sideboarding.

renatino#40340

Similar bug. Sideboarding with no cards in deck and the timer is not running. Also some strange JSON text in the deck name

Hello? Wizards?

Same problem with Lurrus of the Dream-Den as companion.
With the Leyline of the Void in Sideboard, the sided cards were missing