Unread messages in persistent chat rooms are marked as read when you click IM conversation tabs in Skype for Business 2015 (Lync 2013)

Symptoms

Assume that you have multiple persistent chat rooms and instant message (IM) conversations in Microsoft Skype for Business 2015 (Microsoft Lync 2013), and then some messages come. When you click the IM conversation tabs, the chat room messages are marked as read unexpectedly.

Resolution

To fix this issue, install the August 1, 2017, update (KB4011046) for Skype for Business 2015 (Lync 2013).

Note Lync 2013 was upgraded to Skype for Business in April 2015.