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If you run dcdiag and get a failure on the systemlog, it means you have errors in the system event log. You can clear the system event log (make sure you save it) and then your dcdiag should show success for the systemlog.
I ran into that issue today, thanks to petri.co I found the answer and I thought I?d share.
[tags]windows server, microsoft, active directory[/tags]
It’s a well known issue. I wonder why they haven’t put this answer into help/nfo anything like this yet.