Possible causes of "Server not Operational" errors in LDAP - vb.net

Question

I've searched Google for days, but cannot come up with any answers. A week ago, we did a server migration. We have a clustered environment where the following code works on one server, but not the other (and I cannot get it to work on my local machine for our non-clustered development environment):
rootDSE = New DirectoryEntry("LDAP://nonfullyqualifieddomain/RootDSE")
If I try the above, I get the generic error mentioned in the question title (Again, it works on one of the servers, but not the other).
However, when I do this the DirectoryEntry object is instantiated:
rootDSE = New DirectoryEntry("LDAP://fully.qualified.domain", adUserId, adPassword, AuthenticationTypes.Secure)
Based on what I see online, my best guess is that it has something to do with permissions or configurations, but I'm not familiar with the server administration side of the application. Any suggestions would be appreciated!

Answer 1

Every time I've got the "Server not operational" in one of the infrastructure I worked on, it was because I was trying to connect an LDAP server on a bad adress. This was due to :
Bad DNS resolution
Bad Netbios resolution
Firewall filtering
My advice it to use DNS resolution for production and test environment, and to use direct IP adress for development environment.
Be sure 389 (or 636) port is binded on the adress you use and you can connect to it. On Windows Server 2008 LDP.EXE is a good tool to test LDAP connexion (present in W2K3 ressource kit). On development computer I use Apache Directory Studio which is also a good tool to test Directory connectivity and content.

Answer 2

I have come across the same error in one of our legacy applications. This is a memory leak issue. To work around this, I made sure that the code was disposing of the Directory entry once done using it. Something like:
using (System.DirectoryServices.DirectoryEntry OUEntry
= new DirectoryEntry(domain, userName, userPassword))
{
// your code here
}