Yeah, this is still happening. Occured during a Traditional Draft for me.

Happened to me during duskmourn sealed event. Very frustrating that a paid event has these type of issues...

UPDATE, RESOLVED MY ISSUE: It was a deck issue for me. That explained why it happened across all my devices. If you export your deck, check the text file for any cards with the "A-" next to the name for the Arena version. I was playing Explorer where Arena cards are not legal, and Knockout Blow looked normal from my sideboard, but according to the export it was the Arena version. This conflict broke my entire sideboard, regardless if Knockout Blow was sided in or not. It creates some kind of validation issue and the game doesn't know what to do, so it doesn't let you sideboard.

Happening almost every match I play in Pioneer. If I reinstall Arena, it will work normal for a match or two, then revert back to being broken. Happens across all my devices. I've tried re-creating my deck, but no change.

Happened to me on android. Can't do any sideboarding.
Version: 2024.41.0.2908.1091998

Just happend to me as I am 2 wins from mythic playing midrange vs control. There is ZERO chance of winning game 2 and 3 without sideboard. Thanks for the free loss. I’m fuming disable best of 3 or fix this insane ********.

Just happened to me in Duskmourne draft. Arena crashed during first game, and then I was unable to sideboard after game 1.

Same issue on PC. I feel this almost always happens when I concede the first round.

Had this issue happen to me as well. Not sure what caused it as previous games it was working fine.

Only difference is that this time my computer crashed during the game and so I logged in on my laptop instead, after which the error occurred.

Same happenned to me during Metagame Challenge earlier today, it's really annoying ans could have caused me to lose the challenge. I have a screen recording on when I managed to get back after short disconnection and not being able to come back

Wotc tooo busy collecting money and being **** stains to fix even the most simple ****, devs theese dyas suck ***, opposite of essential workers, death to all of them

Same happened to me. The sequence was:
1. In game 1 got a short disconnection. Since Arena NEVER manages to reconnect, had to Alt+F4 and launch the game again.
2. Sideboarding after game 1 and after game 2 doesn't let me add cards, can only remove.

This has happened to me in mobile just now.

Plzz fix

Can't sideboard at all im most bo3 games. It's been ongoing at happens on two different PCs, both windows.

Same issue on mac

Having this issue as well

Other notes - I play on multiple screens and was doing something on other screen when g1 ended
Restarting client or changing the view does not resolve this

Still an issue on PC, BO3 is unplayable

Cant sidebord on Windows

I also cannot sideboard. Android and it always happens after my game crashes and I need to re-enter. Crashes during sideboards are getting very common as well.

Cannot sideboard on iOS