Unable to edit existing Group Policy-The process cannot access the file because it is being used by another process

I have a domain at Forest Functional Level: Windows Server 2008 R2. Two domain controllers, both up to date with MS patches and both rebooted today.

I am trying to edit a Group Policy directly on the PDC emulator and I am getting this error:"The process cannot access the file because it is being used by another process"

I have followed this thread-> http: //social.technet.microsoft.com/Forums/windowsserver/en-US/6b49da66-5e51-43de-b774-eb824fb801cb/the-process-cannot-access-the-file-because-it-is-being-used-by-another-process?forum=winserverGP and the policy is already set to use any available domain controller.

We are using System Center Endpoint Protection and I have disabled Real-Time protection to test with no change.

I cannot seem to find a locked file using ProcessMon.exe, although that may just be my inability to use ProcessMon.

Using the AD Replication Status Tool I find no replication errors.

I think I have searched through the basics but I am getting nowhere. Where can I start looking next? Any tips?