Simple checks would also be, certificate full chain trust NetScaler internal / external network and vice versa. The callback url if configured or not, usually always have a dedicated callback URL gateway VIP for this. STA order NetScaler gateway and StoreFront, double checking the firewall rules for all items, taking the NetScaler out of the equation, does it work with storefront direct login, after that storefront load balancer, also check the source up / X-Forwarded-For client-ip insertion, enable the delegate gateway authentication in StoreFront, check all the event logs for logon errors, take a nstrace from the NetScaler as well.