Welcome to the forums.
Why not start with the basics and tell us what is the version of adclient you're using on each of your Oracle Linux versions? (adinfo --version)
This should be at least 5.4.2.
Also, since you're using users.allow, I'm assuming you are a commercial customer, therefore you can go straight to opening a support ticket.
Note: AD time sync is completely optional. Note that this is a Kerberos requirement as a compensating control against replay attacks, so even if you chose not to sync your systems with Active Directory DCs, you still have to keep time universally within 5 minutes of your time source. Using AD just makes this plug and play.
R.P