Hi Kobel,
Did you solve the issue already?
If the issue persists, please check if there is any Lync related error message on both client\server side Event Viewer.
As the issue happen on the Lync client which hosted on the XenApp 6.5 HRP6 Servers, please check if the issue happen on multiple XenApp 6.5 HRP6 Servers at the same time or not.
Also check if the issue happen at a random time or at the special time period.
When the issue happen, check if there is any network issue between DC and the Lync Server.
Best Regards
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Eason Huang
TechNet Community Support
