Contents of 'Microsoft Windows Network' does not appear

I recently added a second DC (domain controller) to an existing internal
domain, and transferred PDC emulator and such stuff to the new server.

Everything seemed to work ok, until we tried 'map network drive' on a
client. The 'Microsoft Windows Network' should show the domain (we have only
one) and its clients. Instead, nothing happens and the client keeps waiting.
After a long time, a message 'access denied' appears.

If I shut down **either** of the 2 DC servers, everything returns back to
normal.

dcdiag and netdiag on both machines reveal no problems.

Any suggestion welcome!

Paul