Leading cryptocurrencies sank further Sunday, erasing gains made from the relief rally in the days prior. What Happened: Bitcoin fell below $56,000 again, following an optimistic rally that saw the digital asset rise well beyond $58,000 the night before. The second-largest cryptocurrency, Ethereum, also dropped below $3,000 after a temporary respite. The market has been under stress, exacerbated in part by planned Bitcoin sell-offs by defunct cryptocurrency exchange Mt.Gox this month as part of its repayment obligations to its creditors, and potential sales by the German government. The slump …