When running DCDIAG, the utility reports that the SystemLog test failed. This is displayed in the figure below, in which you see domain controller named HUDSONS reported as failing the test systemlog. Notice the reported failures prior to the systemlog failure, those were the errors found in the System Log of the Event Viewer application.
If there are errors in the System Log in the Event Viewer, DCDIAG will report a failure for the SystemLog test.
Check the Event Viewer (eventvwr.msc) and review the entries in the System Log. After you have either resolved or decided to ignore those errors, archive the event log and then clear it (or if you don't care to save the events, simply clear the log).