"An operations error occurred" from DirectoryServices

I'm trying to use System.DirectoryServices to query an Active Directory server, using LDAP. My code can (apparently) connect and authenticate, but when my first DirectorySearcher FindAll() method is called, I only get:

[DirectoryServicesCOMException (0x80072020): An operations error occurred.]

Picking apart the exception in the debugger has been remarkably unproductive. It seems to have pretty much no other information associated with it. The online docs aren't helping me, either.

I'm sure it's probably something I'm doing wrong, but what? Is there any way to tell what kind of problem this is?