There certainly must be more to this story. Big Seller with 1,500+ reviews; bothered about 1 negative review; Fiverr casually instructed OP to liaise with Buyer to initiate mutual cancellation; And then, without warnings, Fiverr descended, banning OP permanently because he refunded Buyer; OP raised tickets to Fiverr and simultaneously opened another account. . . .
This story doesn't just add up as there are missing links here & there. I know there's more to this that OP isn't telling.