From the site,
[Update June 24 - 02:56 GMT] Pushing until 15:00 GMT.
We have found that balances on some older accounts look significantly incorrect when compared with the old database. At this time we do not know what caused the balances to be off, or how many of the older accounts are affected. We haven't touched the old backend or database, so we're going to import the accounts again once we have found what caused the off-balances.
No money is lost, we still have the records of all transactions that happened prior to the bitcoin sell-off. All funds deposited are still in our bank accounts, and we're going to try and get this resolved by 15:00 GMT.
We look forward to getting everyone trading within an hour of getting users logged in and accessing their accounts.