I got curious with your case. I'm not really sure what's the particular issue. You've opened an account with them, passed KYC, made a deposit.

What does the withdrawal failure or rejection message say? Did you check your email for more explanation why your transaction was rejected?

This might have nothing to do with the withdrawal address belonging to a gambling site as others have hinted because you did the same withdrawal attempt with a Coinbase address it was also rejected. If it is about the hold time that DaveF mentioned, then they should've just pointed that out instead of asking for proofs that the address you're withdrawing funds to is the right one and indeed yours. This is the one that made me really curious. Normally, exchanges don't care whether the withdrawal address you're putting is the right or wrong one or whether that belongs to your or not. For as long as it's a valid address, the transaction would proceed.

Anyway, PM CEX and I hope they will reply. Although it last posted in April, it was active until yesterday. And in its last post it says: