The 6.5.1 release is a bit before my time, but in general, that error indicates that a transition document has arrived for a particular step and the associated process instance has already completed (successfully or in error).
Without seeing the process itself, I’m just shooting in the dark, but it’s possible that your process pattern is such that an error transition was taken and the process status was set to “Error” – thereby completing the process. Meanwhile, incoming transitions arrived trying to execute other steps. The PRT is refusing to execute steps for an instance that is done. Again, it’s difficult to know if this is working as designed w/out seeing your process.
You could verify this behavior by looking at the process instance in Monitor and verifying that the status is either Complete or Error.
Hope this helps.
Tom
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