I have ePO 5.1 installed on Server 2012 R2. Initially, I was having the same AD Sync everyone else has been having but got it working by configuring the LDAP server (not a requirement) as well as the System Tree AD Sync settings with the yourdomain\administrator credentials and without SSL connection selected. I'm unsure why this is what made it work (instead of a lesser-privileged account, for security reasons) but this sure helps stop the hair pulling.
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