After three days of email exchange, this is what I got:

Dear customer,

First of all, we would like to apologize for the inconvenience caused!

Unfortunately, your exchange payout has been declined several times by our sending wallet. We had to repeat the payment several times in order to send you funds. This is the reason for the delay.

They refunded me with -1% of what I gave them. I am frustrated because there is nothing that we can do to prevent this happening again.