XenApp - 7.15-Citrix Studio shows the User Still Active but the end user is disconnected from the SVDA. In this scenario active user on the SVDA are not affected but session re connection and new sessions fail to be established on the SVDA UNLESS Citrix Admin has an active ICA session in the same server affected by the behavior, so he can restart the Desktop Service; The active session in Studio shows a "-" (dash) instead of username

Removing Microsoft KB4033428 resolves the issue.

If issue re-occurs, we recommend opening a case with our XenServer team to troubleshoot the issue with server becoming unavailable via console for 20 to 40 minutes if Citrix admin doesn't have an ICA session running on the same server as the users affected.

Problem Cause

Microsoft KB4033428 for 2012R2  is negatively affecting XenServer tools and Server VDA's new sessions or session reconnections.

Microsoft hotfix - https://support.microsoft.com/en-us/help/4033428/windows-server-2012-r2-processor-generation-detection-reliability-upda