Hint: Guessing, in your particular case it could have been that you e.g. here cancelled an order
14.07.2008 16:06:36|1|32|Order='2183335/73674882!RCG!Mirus' Name='Stop loss' New State=PendingCancel Instrument='FDAX 09-08' Action=BuyToCover Limit price=0 Stop price=6281 Quantity=1 Type=Stop Filled=0 Fill price=0 Error=NoError Native error=''
... but the cancellation was not yet effective at the risk management system as you placed your next order here:
14.07.2008 16:06:36|1|32|Order='2184316/73674882!RCG!Mirus' Name='Buy' New State=PendingSubmit Instrument='FDAX 09-08' Action=Buy Limit price=0 Stop price=0 Quantity=1 Type=Market Filled=0 Fill price=0 Error=UnableToChangeOrder Native error='Rejected at RMS - Available buy margin exhausted'

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