‘Transaction Malleability’ bug
The exchange announces the reason for the suspension of withdrawals is the detection of unusual activity on transactions from the exchange’s Bitcoin wallets to external wallets, known as “transaction malleability”.
An individual could request bitcoins from an exchange or wallet service, alter the resulting transaction’s hash before inclusion in the blockchain, then contact the issuing service while claiming the transaction did not proceed. If the alteration fails, the user can simply send the bitcoins back and try again until successful.