The log entry as displayed by the SyncToy UI was empty. No text was in the viewing panel. At the same time, I could go look in the SyncToyLog.Log file and see that it was not empty. That may or may not have been related to the long running job.
I tried to reproduce the problem today and was unsuccessful. Even though a job has been running for the last 12 hours the log is currently visible. I'm not sure how to see the problem again.
But thanks for your comments.