The subject behavior occurs because of enhanced security features intoduced with SP4.

To work around this behavior, revert to SP3 mode for the session lock down feature:

1. Copy / Paste the following to a ObjectSecurityMode.reg file on your server:

REGEDIT4

\[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\]
"ObjectSecurityMode"=dword:00000000

2. Merge the ObjectSecurityMode.reg file with your server's registry, or run regedit /s ObjectSecurityMode.reg.

3. Run the Run As feature from the Terminal Server console, instead of from a remote session.