Its definitly illegal. No flank player. But as you now see. Sometimes this formation is hard to beat. In my eyes the hardest illegal formation.
I i were you. I had played with 4-5-1 V. Normal mentaly. Red arrows on dl, dr, ,aml, amr, ,striker.
Blue arrow on dmc. No counter. So you have your attacking players and your outside defenders to go to the weak point of this formation.
But may be since this was a homematch you have to play attacking mode. But this you can see during the match.
I feel your pain Emiron. This is a bad result against a illegal formation. But hope will never die. You have still the chance to go to the next round.