While trying to demote, dcpromo out, a domain controller the operation might fail with the following error
"The directory service is missing mandatory configuration information, and is unable to determine the ownership of floating single-master operation roles."

As you can notice, in my case, the partition at problems was DC=ForestDnsZones,DC=domain,DC=int.
To investigate more the problem I issued a dsquery command
dsquery * CN=Infrastructure,DC=ForestDnsZones,DC=domain,DC=int -attr fSMORoleOwner
The result clearly shows, as indicated in the event viewer, that the fSMORoleOwner is set to an orphaned object CN=NTDS Settings\0ADEL:xxxxxx

Now that we know what is the problem let's solve it.
I opened ADSIEdit.msc and connected to: CN=Infrastructure,DC=ForestDnsZones,DC=domain,DC=int

right click -> Properties on "infrastructure" and looked for the fSMORoleOwner attribute and remove the \0ADEL:xxxxxx from CN=NTDS Settings\0ADEL:xxxxxx. In my case the domain controller was still active it is just the \0ADEL:xxxxxx part that was wrong.


All looked pretty good, however when I tried to apply the changes I was faced with another error "The role owner attribute could not be read".

The solution to that last bit was to repeat all the steps while connected to the domain controller holding the schema master role instead and voila I was able to demote the DC without any issues.
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