504 Gateway Time-out 
Yes, here, too. Seemed like my GPUs were still hashing accepted shares, but I switch to ANC pool for the time being. I'll keep checking back.However, to know what's going on look at cgMiner and press "P" to see your list of pools and their Dead or Alive status. MP seems to be dead at the moment. cgMiner will continue to poll it and when it goes live it will switch back.
Always specify "failover" pools as a safety back up.