Thank you logfiles.
I can see that Italy is restricted. It is my bad, I only checked the Terms and Conditions and some other information links but I missed that one.
Still, as pointed out by Fivestar4everMVP, the casino asks for personal data upon registration and I immediately declared that I was from Italy. That didn't stop them from making me register and deposit my money.
I can understand the freezing of any winnings but I think that to maintain even a shred of reputation the casino should return my deposit.