>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 .

I finally found it. This was Windows Firewall which was enabled on those servers. Basically, not all those switches were off. There is the Domain Profile is Active, Private Profile and Public Profile.

So, that resolved two more.

I still have two left, but those are domain controllers. There seems to be something else to bypass in there to be able to connect in RPC.

On those ones, adding the specific account as part of the Administrators group is different. It doesn't have it in the Administrative options. We have to go in Active Directory Users and Computers under Builtin. It might be related to something related to that.