Outlook "Trying to connect" ,OWA & mobile users are able to connect | Exchange 2013

Hi ,

I have been experiencing a strange problem lately , all of my outlook users get disconnected and goes to "Trying to connect"status come back online after i restart cas servers. meanwhile owa & mobile users are able to use their emails without any disconnection. I have been searching for the logs but i haven't found any solution yet . Please help me to identify the issue. server address is pointing to Windows NLB interface CAS servers.
also i have checked port 443 is not blocked by Firewall.

PS:- I'm using Microsoft exchange 2013 Enterprise ,2 cas servers & 2 mbx servers on  windows server 2012
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Sharaf KEXchange 2013 admin & Network AdminAsked:
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SunilTechnical Specialist - Wintel Operations.Commented:
Sunil ChauhanConsultant Commented:
that would probably happen when the RPC service is going into hung state, monitor the server resource utilization.. check for mailboxes with the highest number of the folders and items.. that could be one the reasons....
Robinsan ShawSenior Data Recovery ExpertCommented:
To troubleshoot RPC service, you have two methods:
1 Troubleshoot RPC service from Windows
2 Update or create your Outlook profile with RPC encryption
For troubleshoot RPC service from Windows follow steps below:

Method 1: Monitoring Firewalls
Method 2: Check Internet Connection and its protocols
Method 3: Verify RPC (Remote Procedure Call) Services

For Complete Steps Detail visit: https://www.experts-exchange.com/articles/32019/Methods-to-Troubleshoot-RPC-Server-is-Unavailable-Error-in-Windows.html

To  Create and update your Outlook profile with RCP encryption click on the link below

https://support.microsoft.com/en-in/help/3032395/outlook-connection-issues-with-exchange-mailboxes-because-of-the-rpc-e
Sharaf KEXchange 2013 admin & Network AdminAuthor Commented:
I also noticed that i get below error quiet frequently.

Event 36888,Schannel

A fatal alert was generated and sent to the remote endpoint. This may result in termination of the connection. The TLS protocol defined fatal error code is 10. The Windows SChannel error state is 1203.

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This is all started all of sudden when owa,ECP,outlook& powershell was not accessible few weeks back. The reason been SSL certificate binding was lost on IIS backend & frontend Website.
and i followed below link to rectify.but since then once in a few days just the outlook users goes to "Trying to connect" state .(meanwhile OWA  mobile users can access email without any issue) .I just cant figure it out what it is.

https://support.microsoft.com/en-us/help/2971270/blank-page-after-login-exchange-eac-owa-ecp

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Sharaf KEXchange 2013 admin & Network AdminAuthor Commented:
Update:-Instead of restarting CAS Server ,Restarting Microsoft exchange health manager service will let outlook to connect to exchange
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