VERIFIED SOLUTION i

Resolve the Mail360 Server User Interface (UI) error that 'the underlying connection was closed..."

Issue

Installing Mail360 Server the first time, using the User Interface (UI) getting an error that 'the underlying connection was closed...'

Cause

User was looking at the original MAIL360 Server installation instructions, dated June 2010, and everything seemed to go correctly up to a point. 
Then we compared that old document to the newer one dated October 2014, which is the MAIL360 Server 'Revision A' Installation Guide, and right at the end there is a new section "Importing and Configuring SSL Certificates" that says that you have to bind the SSL Certificates to the port (default-8882.)
The user had not done that since the step was not in the original Installation Guide he was using.

Resolution

UPDATED: March 22, 2017
Once the bind was done to the SSL Certificates, the software worked ok.
Insure that the current 'Revision A' Installation Guide is being used.
(And it has other helpful changes in it besides the specific one mentioned in this article.)
Environment Details