think its a bug or hack or something

[Potemy]

It means you played without any wingers,or with less than 3 defenders.

There are more rules besides this, same goes to Michael. And Michael, he did much more wrong than just playing with a stupid lineup, if he had played a legal formation, even if the opponent found his weak spots, I believe he would have won easily.

I think it's the same as (5-2-3) which i played last season for just 1 game(cup semi final), which I have 20% possession only as well. Opponent Quality is Half of mine and he was playing 5-3-2. It's kinda weird as these were also "considered" as illegal? That lost of first league cost my cup.

DR DC DC DC DL
MC MC
ST ST ST

I would say you need at least 3 midfielders, that's all we can speculate here. It's worth noting that 5-2-3 is the only possible formation with less than 3 midfielders.