This was due to permissions. The post https://serverfault.com/questions/731511/windows-10-ad-domain-admin-with-missing-rights outlines the steps required to fix this issue (the title was somewhat misleading). The answer provided by HEDMON to that post solved this issue :
- Win + R and type 'secpol.msc' for open the Local Security Policy console.
- In the Security Settings tree, open Local Policies > Security Options.
- Find the policy: User Account Control: Admin Approval Mode for the Built-in Administrator account and enable it.
- Log out - log in, voilá!