...and here's the log from the DCDiag program (replaced real domain name with mydomain.com);
=============================================
DC Diagnosis
Performing initial setup:
*** Warning: could not confirm the identity of this server in
the directory versus the names returned by DNS servers.
If there are problems accessing this directory server then
you may need to check that this server is correctly registered
with DNS
[10.1.16.2] Directory Binding Error 87:
The parameter is incorrect.
This may limit some of the tests that can be performed.
Done gathering initial info.
Doing initial non skippeable tests
Testing server: Default-First-Site-Name\PDC_RICH
Starting test: Connectivity
PDC_RICH's server GUID DNS name could not be resolved to an
IP address. Check the DNS server, DHCP, server name, etc
Although the Guid DNS name (5baaee45-4f08-4c8f-8e2d-ad13dabe732c._msdcs.mydomain.com) couldn't be resolved, the server name (pdc_rich.mydomain.com) resolved to the IP address (10.1.16.2) and was
pingable. Check that the IP address is registered correctly with the DNS server.
......................... PDC_RICH failed test Connectivity
Doing primary tests
Testing server: Default-First-Site-Name\PDC_RICH
Skipping all tests, because server PDC_RICH is
not responding to directory service requests
Running enterprise tests on : mydomain.com
Starting test: Intersite
......................... mydomain.com passed test Intersite
Starting test: FsmoCheck
[PDC_RICH] DsBind() failed with error 1722,
The RPC server is unavailable..
......................... mydomain.com passed test FsmoCheck