If you ever get authentication failures when trying to log on to NetScaler Gateway with credentials you know are correct then start logging the authentication attempts on NetScaler using aaad.debug to find out what is going wrong.
citrix
Citrix Insight Services
Citrix Insight Services (CIS), formerly known as Tools as a Service (TaaS) is a set of tools available online focused on making the support of your Citrix environment easier. CIS does this by collecting information on your environment, analysing that data and responding with recommendations for you to make to that environment such as hotfix patching recommendations or configurational changes.
NetScaler index.html corruption
Recently upgraded NetScaler 10.5 to 11. Upgrade completed fine but when accessing the NetScaler Gateway portal nothing other than the below was presented. No logon screen. Just a blank white page. Same result using multiple web browsers. This also happened on a seperate occasion when there was a power cut that affected a NetScaler MPX.