When trying to log on to the NetScaler using the GUI or the NITRO API, external users (from LDAP, TACACS, and so on) get the following error message:
"User does not exist"
To resolve this issue upgrade to NetScaler software release 10.5.52.11 or later.
A bug in NetScaler software build 10.5.51.10.
Citrix Discussion - 10.5 build 51.10 GUI LDAP Authentication bug
Refer to issue ID 498221, 501681 in NetScaler 10.5 Build 52.11 Release Notes for more information.