You see sessions with no current user and associated user name in Citrix Studio as below.



Though Citrix Studio shows the session, actually, it doesn't exist on VDA server.
Only the session information is created and remain on SQL database.

 

If it's Server VDA, restart "Citrix Desktop Service". It will not affect existing connections.
If it's Desktop VDA, reboot the VDA. (usually rebooting "Citrix Desktop Service" doesn't work)

Or

Remove the VDA from Delivery Group and Add the VDA again. ( This is applicable both Server VDA and Desktop VDA )

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Problem Cause

This issue happens if something happens in early state of session creation and so the session is not established completely. 

The reason of the session creation failure is as below.

1. session was not established as they have problem on RDS CAL.

2. session was not established as they have problem on user profile.

3. session was not established as they canceled during the session creation

4. session was not established as they canceled password change during the session creation.

Note: We may have other scenarios of session creation failure.
 

https://support.citrix.com/article/CTX205512 
https://support.citrix.com/article/CTX217246 
https://support.citrix.com/article/CTX233920 
https://support.citrix.com/article/CTX237423