Same issue - so far, only tested playing any previously-saved deck during a challenge match with a friend. No matter who hosts, the submit error occurs. If I am not hosting, it loads the profile page and crashes on the host's side. If I host, shows the opponent the profile page and then gives me the deck validation error. No need to attach a log file since this issue is far more than a local problem on my machine.

Getting a Deck Validation Error on a Torbran Brawl Deck. No Mirage lands in the deck and it looks like everything is legal. Here's the list - could I get some help here? Thanks!

Commander
1 Torbran, Thane of Red Fell (ELD) 147

Deck
1 Anax, Hardened in the Forge (THB) 125
1 Bonecrusher Giant (ELD) 115
1 Careless Celebrant (THB) 129
1 Castle Embereth (ELD) 239
1 Chandra, Acolyte of Flame (M20) 126
1 Chandra's Pyrohelix (WAR) 120
1 Cosmotronic Wave (GRN) 95
1 Cryptic Caves (M20) 244
1 Dwarven Mine (ELD) 243
1 Ember Hauler (M20) 137
1 Embercleave (ELD) 120
1 Experimental Frenzy (GRN) 99
1 Fervent Champion (ELD) 124
1 Fire Prophecy (IKO) 116
1 Goblin Banneret (GRN) 102
1 Krenko, Tin Street Kingpin (WAR) 137
1 Lava Coil (GRN) 108
1 Legion Warboss (GRN) 109
1 Light Up the Stage (RNA) 107
1 Mobilized District (WAR) 249
20 Mountain (IKO) 271
1 Ornery Goblin (GRN) 112
1 Phoenix of Ash (THB) 148
1 Purphoros's Intervention (THB) 151
1 Rimrock Knight (ELD) 137
1 Risk Factor (GRN) 113
1 Robber of the Rich (ELD) 138
1 Runaway Steam-Kin (GRN) 115
1 Scorch Spitter (M20) 159
1 Scorching Dragonfire (ELD) 139
7 Seven Dwarves (ELD) 141
1 Skarrgan Hellkite (RNA) 114
1 Slaying Fire (ELD) 143
1 Spear Spewer (RNA) 117

I just put together a boros cycling deck, spent my wildcards to do it too, and got a deck validation error. Is there any way to fix this? I'd hate to think I just wasted all those wildcards...

Just received the error

I'm having the same issue with Pauper FNM.

Bug still exist...

I have the same error playing Historic Pauper. I have the following line in the log:

Error response for message. {"id":62,"code":6900,"message":null,"data":"{\"eventName\":\"Historic_Pauper_20191128\",\"validationErrors\":[\"Too many copies of a specific card: titleId 763\"],\"deckName\":\"Black\",\"format\":\"HistoricPauper\"}"}

Same for me... When trying Historic Pauper, I have a deck validation error. i do not have any mirage land in my deck.

steps to reproduce the bug

1. Create a new deck
2. Add Basic Lands from set Mirage (plam tree)
3. Add more cards to create a valid deck
4. Start a game and select the new created deck

The actual results you’re experiencing:
A Error Message network error - deck validation failed is displayed

The expected results of the feature:
Start a match with selected deck.

workaround:
Replace all basic land from set Mirage (Plam Tree) against basic land from another set.

I had this same issue, looking at the logs it said one of my cards was throwing a validation error, with an invalid card ID. I went through and started pulling cards out to see what could be throwing the error, turns out it was my John Avon Mirage Mountains (supposedly, card grpid 7153). I swapped them out with different mountains, and got in no problem.

Also happens to me, but not all decks have this problem only a few. Affected decks are legal for the format that they were meant to play in, contains at least the minimum amount of cards required.

happening to all my custom decks...

Found the problem.. White land, the first one (0) on the land choosing, replace it for another one

After a large patch download today (21Nov 2019) I keep getting a 'deck validation' error when I try to play my embercleave deck in any game mode, my other decks work fine and it doesn't say the deck has any issues in the deck builder. I haven't changed the deck since I played with it a couple of days ago. Also all the graphics are now extremely low res, even after I set the graphics back to Ultra.