Check that the DNS entry on the NAS-R3 has the IP of the ADS server (you can add more with the semi colon). Also review these links that are related to the this issue.
The Active directory server will synchronies with the Open-E machine every 5 minutes.
RPC may not work when trying to go between different subnets, This is dependent on your network. please test with ADS server and Open-E server in the same subnet.
After ADS reconnection, it may take a while to generate a new Kerberos ticket
We do not support Multiple Domains
We recommend to add the administrator account to the super users list (Configuration->NAS settings->SMB settings), and recommend to set all permissions from the AD side.