“Your deck is not valid for this format.” message sent to both parties, twice, after pressing play and the rolling animation has gone for a while. Of course, both of your decks are perfectly valid.
This started around the time you added ETC to the game (day after). It’s been around since then -still not fixed- and appears every day, multiple times a day, when playing friendlies, with multiple friends, on either side. Phone/computer. To fix it, one might need to spam select deck/play match 5-20 times before it actually works, often you need to change deck to save time/it never works. Sometimes, if you borrow decks (and play your own), it fixes that, but not always. Rechallenging again and again does nothing to fix it.
Now one can live with that, it’s just a minor annoyance every single damn day. But might as well say it out now that I’m sending bug reports…