Safely Demote a Domain Controller?
Solution 1:
If your other DCs don't have SYSVOL shared out, then find and fix the problem with your NTFRS or DFS-R. Maybe you had a rollover. You should have events related to the problem, unless they've aged out of your event viewer.
REPADMIN won't show this, it's only AD replication and doesn't have anything to do with SYSVOL replication.