All good now.

So, to explain: to withdraw, you need to enter your account password as well. It seems I forgot mine, and the site returned a message saying "Withdraw Failed" instead of saying "Invalid Password".

In the end, I have reset my password and tried again. ETTA Games offered to pay back the mining fee after all of this.
I'd say this was just unfortunate and I'm happy everything turned out okay.

I will probably test the site again in the future. Note: I'm not a big gambler or anything , but once in a while I like to test new sites/concepts.

All the best!