RPC unavailable

RPC unavailable

Is there a definitive guide to fixing the dreaded "RPC Unavailable" error message. 
There are a number of guides on the internet but some of them relate to other ManageEngine products and few specify the version of Windows OS that is applicable.
On our estate, we have some systems that work and some that don't.  A line by line comparison does not show any differences. Disabling Windows firewalls makes no difference. 
We are beginning to lose faith in ELA as its becoming a lot more trouble than its worth.
Does anybody have a checklist the can share.