Some Users cannot login via Lync 2013 windows client

Question

Hi,
I have a Lync 2013 FrontEnd server, DB server and Edge Server.
Since last noon we are facing a mysterious issue. Its as below.
( I have just entered into Lync administration)
User A and User B, both were able to login to lync till yesteday noon on their individual PC, any PC or mobile (windows, android, mac) with any version of Lync client, inside and outside the organization.
Last noon User A reported that he is facing an issue that whenever he logs in to lync 2013 client he is automatically logged off in 2 to 3 seconds. Then again logs in automatically and again loggs off in 2-3 seconds. and thats all is happening again and
again. The message he is is getting after he is logged off is "The connection to the server was lost
. Reconnecting. Current calls may continue, but with reduced functionality".
But at the same time he was able to login from other computer and his mobile device.  At the same time, User B is not facing any issue on his machine.
Then User A tried to login from the User B's computer and there also he faced same issue. Then i asked User B to login on computer of User A and what i see is User B is able to login on User A's computer.
What is concluded from the observation is, User A cannot login from lync client 2013 from any computer inside and outside of my network and can login from any other client/version/platform inside or outside of my network.
What i checked for is Client Version Policy of User A and User B, but both are same (Automatic). Below is my client version policy and i am using client version 15.0.4605.1000
In Sing In Logs. (By Right click on system tray icon of lync) i got below information.
Error-1
In Windows 8, event viewer i am getting only Event ID 1 or 12 which says, "
The description for Event ID 12 from source Lync cannot be found. Either the component that raises this event is not installed on your local computer or the installation is corrupted. You can install or repair the component on the local computer.
If the event originated on another computer, the display information had to be saved with the event.
The following information was included with the event: 
Lync
Thanks, Rishi Pandit.

Answer 1

The issue has been resolved by removing an entry from "Trusted Application Server". This application server was being used for Polycom VC infrastructure integration with Lync.
As per Task 4 section in
Polycom and Lync Integration Documentation we have to use below-
http://support.polycom.com/global/documents/support/strategic_partner_solutions/Polycom_UC_Microsoft_Deployment_W8.pdf
Use the New-CsStaticRoute command to set up a static route for the RealPresence 
Collaboration Server system. 
$route = New-CsStaticRoute -TLSRoute -destination rmx.corp.local 
-port 5061 -matchuri sipdomain.com -usedefaultcertificate $true 
where rmx.corp.local is the FQDN of the RealPresence Collaboration Server SIP signaling 
domain and sipdomain.com is the name of the Trusted Application Pool you created. 
but this was wrong.
we used sip.domain.com and issue got resolved. Polycom forgot one (.) dot between sip and domain.com which ruined us.
Thanks.
Thanks, Rishi Pandit.