The following error is shown after logon to NetScaler page:
Error " Http/1.1 Internal Server Error 43531"
The NetScaler is unable to connect or redirect to StoreFront page.
Verify the StoreFront URL. In case of HTTPS store, Verify the certificate on StoreFront servers.
You should have the valid certificate for the Base URL and Base URL should be along with FQDN not anything like only hostname or something.
Example: StoreFront.Domain.com
Refer to Citrix Documentation - Configure NetScaler Gateway connection settings