The RPC server is unavailable. (Exception from HRESULT: 0x800706BA)

I am having an issue with one cluster of servers connecting to an LDAP server to update some Active Directory users.

I am working on a legacy .NET application. There are two separate environments (server clusters) I am working with for this application: Integration and Certification. When running the application in the Integration environment, I have no issues. When running the application in the Certification environment, I receive the following error when trying to invoke a command on the LDAP server:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Runtime.InteropServices.COMException: The RPC server is unavailable. (Exception from HRESULT: 0x800706BA)

Some things to know about this issue:

• The code for the application is identical in both environments. I have verified this numerous times.
• Both environments touch the same LDAP server

I work for a large company and have absolutely no visibility on the servers in either environment. I have worked with a few people on our infrastructure team (who have not been very helpful) and here is what I had them try on the Certification servers:

• From command prompt, run: telnet LDAP_SERVER_NAME PORT

  • Result: The Certification servers successfully connected to LDAP_SERVER_NAME on port PORT

• From command prompt, run: ping LDAP_SERVER_NAME

  • Result: The Certification servers are able to successfully ping the LDAP server

I have worked with numerous different people on this issue for over a month and no one has been very willing to help me, and I'm unsure of what I should do from here. They are telling me that it's certainly not a firewall issue, and certainly not a server issue, so therefore it must be an application issue. However I don't see how that's possible if it works totally fine on one server cluster and not the others.

My real question here is, what test(s) should I perform next to determine what the cause of this error is? I don't want to waste more of our infrastructure team's time, but I need to get this error figured out.