In case you see this error, that shows up right when you open the aaa or gateway vserver (before logging in), this is the solution: https://support.citrix.com/article/CTX244520

When the “Integrated caching” feature isn’t licensed, you will just need to execute the following command:

set aaa parameter -enableStaticPageCaching NO

I tested it on Netscaler ADC 12.1 51.19 and it worked perfectly.