We have a customer whose invoices sometimes aren't making it to GP. Per Microsoft, they have different versions of the eConnect incoming & outgoing service enabled. See the following screen shot:

When the process fails, they get 3 errors show up in the event log.
The event ID for all three is 906.
The message for each is slightly different.
Message 1:
11.0.0.0
System.Messaging
System.InvalidOperationException: Property Label was not retrieved when receiving the message. Ensure that the PropertyFilter is set correctly.
at System.Messaging.Message.get_Label()
at Microsoft.Dynamics.GP.eConnect.Incoming.eConnect_Incoming.Send_To_Deadletter(MessageQueue q, Message m, Exception Exp)
Message 2:
eConnect Incoming Service Version=11.0.0.0
System.Messaging
System.InvalidOperationException: Property Label was not retrieved when receiving the message. Ensure that the PropertyFilter is set correctly.
at System.Messaging.Message.get_Label()
at Microsoft.Dynamics.GP.eConnect.Incoming.eConnect_Incoming.MyReceiveCompleted(Object source, ReceiveCompletedEventArgs asyncResult)
Message 3:
11.0.0.0
System.Messaging
System.InvalidOperationException: Property Label was not retrieved when receiving the message. Ensure that the PropertyFilter is set correctly.
at System.Messaging.Message.get_Label()
at Microsoft.Dynamics.GP.eConnect.Incoming.eConnect_Incoming.Send_To_Deadletter(MessageQueue q, Message m, Exception Exp)
The process doesn't always fail. Does anyone have any ideas what might be happening here, and how to resolve it?