You have been running dirsync tool to sync your on-premises Active Directory to your Office 365 successfully. One day, the machine reboots or due to some other issues you found that the Active Directory is not syncing any more. So you went to the server that runs dirsync to find out. You check the EventViewer and true enough the dirsync tool throws this error. Or you try to manually run the Directory Sync Configuration Wizard and it fails at the last step with this error, 'the current user is required to be a builtin\administrator directory sync". To resolve this do these steps.
Just like you, I searched everywhere and there are so many suggestions. But the simplest one and the one that works always is to Uninstall and reinstall dirsync.
Just go to control panel and uninstall the microsoft online directory synchronization
Next, login to your 365 online and download the new Dirsync tool, or if you have the exe file already, just run it again.
In the last step of your installation wizard, remember to UNCHECK 'Run configuration wizard now' and then close the wizard
Now restart the server
After installation, right click the Directory synchronization icon in your desktop and select 'Run As administrator'. Follow the steps and you are done.