I got this same exact reply when I transferred some tokens to my Coss exchange wallet and the tokens never arrived in the wallet, this is a thinly veiled scam.they call an exchange, I would never trade there in my lifetime again.
What's the minimum amount to deposit your COSS token? IIRC there is a minimum which you need to follow in order to make sure the balance of the tokens are reflected on your account. I used COSS before and except for its higher fees than average exchange and the slow deposit/withdrawal, there is no further issue.
The fees are significantly reduced with the new market maker and negative fees program