Today I had an issue where I received "Access is Denied" when disabling SID Filtering, even though my account had access.
Of course I checked my command prompt was running with administrative rights... check this is you have User Account Control (UAC) enabled.
I ran the netdom command with /quarantine:no and the correct syntax and received:
The command failed to complete successfully.
To get around this problem I established a UNC connection to the the PDC emulator in the destination forest. This resolved the problem. Please see the screenshot below: