Solution 1:Change to default theme and refresh browser.
Make the UI theme parameter to Default on the NS with issues .
You can do this by going to the Netscaler UI >Netscaler gateway >Global settings >change global settings>Client experience tab >set UI theme to Default and click OK .
Solution 2: SSH into the NetScaler set the portal theme to default by issuing the following command:
set vpn parameter uitheme default
This will set the portal theme back to default, and we are now able to load the NetScaler Gateway login screen.
Solution 3: In case some manual customizations have been applied to NetScaler, it is advisable to use the GUI files from the particular NetScaler version after upgrade.There were some fixes made for IE 11 compatibility which might be missing in the customer’s case as they might still be using old customized GUI files.