>I am using .NET code to connect to RPC to collect some Event Viewer records. The code seems to be find as it works for some servers.
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>I have 7 servers and it works for 3 out of them. This is under a domain. In order for this to work, we need to add the account used for the operation into the local admin group of each server. This has been done. But, there is another rule somewhere that forbids 4 servers to refuse the connection. That returns RPC server is unavailable.
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>Anyone would know what additional rule might be in effect? I have compared side by side, using RDP, the settings of pretty much everything and I cannot find any difference.

There's a fairly comprehensive troubleshooter at https://social.technet.microsoft.com/wiki/contents/articles/4494.windows-server-troubleshooting-the-rpc-server-is-unavailable.aspx .