We’ve confirmed with Poloniex that the suspension of Zcash deposits and withdrawals from approximately from July 7th through July 15th was due to a wallet corruption and performance problems in the wallet implemented by the zcashd (MagicBean) software, under conditions of Poloniex’s large-scale and increasing usage.
The issue did not threaten security of Poloniex user funds because Poloniex had backed-up the wallet. To make sure your funds are safe, see Wallet Backup Instructions.
The Poloniex Zcash wallet is currently operating normally. We’ve offered to support Poloniex by helping them deploy operational workarounds and diagnose the root cause. We’ll post another update once we learn more.