Hello cer113,
Thank you for posting in Microsoft Community forum.
1.Based on the description "when i perform Cumulative update September then all of a sudden domain admin password is no longer working. on DC1 it is all working fine", do you mean for the same domain Administrator, you can log into DC1 but you cannot log into DC2?
2.If so, what error message do you receive when you log into DC2 using domain Administrator?
3.Based on the description "So now I cannot login to computer using RDP", please check can you log into DC2 using domain Administrator locally?
4.The last option is you can try to uninstall the KBs within Cumulative update September and then check if it helps.
I hope the information above is helpful.
If you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou