"An invalid directory pathname was passed"

[Adam Tauno Williams]

I was testing and suddenly the MMC snapin "Active Directory Users and
Groups" stopped working.  Attempting to access it fails with

The directory schema is not accessible because:
An invalid directory pathname was passed.
For this reasin, the New menu may be inaccurate, and extension
snap-ins may not work properly.

- followed by -

Data from Active Directory Users and Computers is not available from
Domain Controller (null) because:
Unspecified error
Try again late, or choose another DC by selecting Connect to Domain
Controller on the Domain context menu.

Login and ADSI Edit still seem to work normally.  And I can still see
schema via the "Active Directory Schema".

The samba.log doesn't seem to contain any 'obvious' error messages.  Has
anyone seen this issue?  (Although with the constant REPLCHANGED queries
the log isn't terribly easy to read - is there some method for
squelching just those messages?)