Troubleshoot Lync Client Login Issues

Co-Author: Vijay A.


This article goes over a walk through of several basic troubleshooting steps for reasons why a Lync client won’t log in.  There are about a billion reasons why Lync may fail to log in, this is one of them.

Initial Troubleshooting:

  • Verify user has an enabled Lync account
  • Verify user can resolve FQDN of Front End Server (Pool)
  • If FQDN does not resolve, verify your DNS settings and make sure client doesn’t have any bad settings in HOSTS file
  • Verify user can telnet to Front End Server (Pool) on port 5061

If the connection from the client to server appears to be successful the following setting has been found to fix client’s that were unable to log in. This setting should be required on the client when 128 bit NTLM SSP Encryption is enabled on the server side.

  • Check the following Local policy on the client computer where Lync won’t log in
  • Click on Start > Run, type gpedit.msc and press enter.
  • It will open below screen.login1image001
  • Now in the right pane expand Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options

  • Double click on Network Security : “Minimum Session Security for NTLM SSP based (Including secure RPC) clients” and “Network Security : Minimum Session Security for NTLM SSP based (Including secure RPC) servers” one by one. It will open the screen similar to the below depending upon the version of your Windows Operating system.

  • By default the “Required 128-bit encryption” box would be showing unchecked. Check this box and now the screen will look like below. After that go to “CMD” prompt and enter command “GPUPDATE /FORCE”.

  • Click on “OK” and then restart your computer.
  • After machine is restarted you should be able to login to Lync 2010 client.



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