On July 19/20 there is going to be a Hard Fork on the ETH blockchain to compensate for the DAO issues according to the Kraken Exchange News Mailout. Users were made aware that deposits and withdrawals would be paused for about an hour next week during the coding update, but that could change depending on the updates. You can stay tuned by checking out the Forum or the Kraken Blog to see how the ETH coin gets affected.
Or, you can read a more detailed article on the subject from Olusegun Ogundeji on Coindesk.
Traders may want to update their deposit addresses and mining addresses after the work to make sure they are working correctly.