Step One: There is an Essentials server test tool made by Microsoft that you can use to determine what underlying issues you may have:
In my case everything passed on the initial tests but I was receiving the error on Ports “6602” and “8192”
I created an additional firewall rule to make sure those ports were open in and out.
The error persisted. I found that other services may be fighting for those ports so I ran the following command:
Set-NetNatTransitionConfiguration -IPv4AddressPortPool @(“serveripv4, 6001-6601”, “serveripv4, 6603-8911”, “serveripv4, 8913-47000”)
See example below with server IP “192.168.1.11”:
I then reran the essentials server test tool and the ports were OK. I then restarted the Essentials 2012R2 Server and all clients started to come back online and backups began to run.
Reference: Fred Moore, Small Business Susan http://blogs.msmvps.com/bradley/2013/03/21/direct-access-and-essentials-go-around-the-port/