Reproducing the Issue
1) Login as an Admin
2) Search for a user you want to proxy.
3) Proxy the user via action section.
4) A new window appears via which you see the the proxied user profile but you will get login screen in this window and as you close proxy session you will get logged out of the system.
This is happening because the user you are trying to proxy is already terminated.
As explained in the cause, as the user you are trying to proxy is already terminated from the system that is why the system is behaving as such.
Now how does the system know the employee is terminated?
If you go in user details page of the user in the summary section
And then check in the employment section you will see there is a terminated date if this date is papulated and in the past the user is terminated.
If still you need to proxy the user you can remove the terminated date and proxy.
KBA , LOD-SF-LMS-ADM , System Admin, Global Variables, References , Problem