Nick,
That is a strange error. The text of the error itself ("missing data set interface") does imply a connection problem. When EG runs a stored process, it attempts to attach a listener to the underlying stored process server/workspace to check whether any data sets are created as part of the stored process execution. This way it can try to make sure those output data sets are reflected in your project.
With a connection problem like this, it's possible that the event listener logic is getting confused because it cannot connect to the workspace session as it expected to.
In general though, "UNKNOWN" identity is going to spell trouble for connection issues downstream. If EG cannot determine who the metadata identity is, then it's likely that other processes will have the same issue.
Chris