Couple of things to look at. There’s no such thing as a PDC/BDC anymore, hasn’t been since NT 4.0. All DCs are equal now. The only small difference is one will have the PDC Emulator FSMO role. (This can be moved between DCs at any time). Which ever one holds the PDCe role needs to be set as the authoritative time source for the domain.

What is the domain and forest functionality level? (DFL and FFL) If your lowest DC is 2012R2 then you can set both of these to 2012R2.

Check your DNS config on your servers, this is the biggest source of issues. See the following topic on that.

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