When you try to sign in to the Skype for Business 2015 (Lync 2013) client in Windows 8.1, you may receive the following error messages:
The requested operation was blocked by policy. For more information, contact your system administrator.
There was a problem acquiring a personal certificate required to sign in. If the problem continues, please contact your support team.
Note This issue usually happens if an admin has set a conditional access policy (the PkeyAuth protocol support) through Intune on the user for signing into the Skype for Business client.
To fix this issue, install the December 5, 2017, update (KB4011284) for Skype for Business 2015 (Lync 2013).
Note Lync 2013 was upgraded to Skype for Business in April 2015.