NetScaler login page is not accessible after an upgrade from the version 11.0 to the version 10.5 . NetScaler login page has customization in place and login page is accessible through Chrome and Firefox.
 

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.
https://www.citrix.com/blogs/2014/01/06/ie11-compatibility-got-you-down