If you have the directory administrator password, then local DNS services and/or Open Directory is likely corrupted, and there's not much point to resetting the diradmin password.
Launch Console.app from Applications > Utilities, attempt the login, and look for any relevent errors.
If this is a NAT'd network, then local DNS services have to be correct, or OD can and variously will fail. To verify local DNS, launch Terminal.app from Applications > Utilities and issue the following non-destructive diagnostic command:
sudo changeip -checkhostname
This command will find most — but not all — DNS configuration errors. Enter an administrative password for the sudo, and you can safely ignore the one-shot warning nessage about using sudo if you see that. You'll get back some information on the current host network set-up, and then an indication that no changes are required, or that there are system or network or DNS errors and some details.