Booking.com Car Rental Cancellation and Refund Problems on Lesbos Island

I rented a car through the Booking.com app for use on Lesbos Island. Initially, the payment was processed, but then my reservation was canceled. I received an email stating that my reservation number 743124849 would be refunded. Trusting this information, I rented the same vehicle again, and this time my reservation was confirmed with reservation number 763930711.
I expected the refund to be processed within 10 business days, but I have not received anything. When I emailed them, they claimed they hadn't received the money and that my bank had placed a hold on it. I contacted my bank, and they confirmed that there were no holds or pending refunds.
After sending another email with my transaction records and receipts, I continue to receive automated responses, and they don’t seem to read my emails. Now, they have stopped responding to my messages altogether. I have attached all necessary visuals, and if my money is not refunded, I will file a complaint with the Consumer Arbitration Board.