Check error logs on StoreFront servers and netscaler

Check if theres any Schannel errors in Log of StoreFront ( Datacollector Servers ) and reboot if so

Check the STA settings on the Netscaler

1. Configuration tab, in the navigation pane, expand NetScaler Gatewayand then click Virtual Servers.
2. In the details pane, select a virtual server and then click Open.
3. On the Published Applications tab, under Secure Ticket Authority, click Add.
4. In the Configure STA Server dialog box, enter the URL of the STA server and then click Create.
   1. This will be http://datacollector or ip
5. Repeat Step 4 to add additional STA servers and then click OK in the Configure NetScaler Gateway Virtual Server dialog box.

Make sure your Citrix StoreFront is configured with the STA details in the NetScaler Gateway section (remember you only need to use the STA in case of remote users, for which you would have to configure a NetScaler Gateway). They need to be the same also clear DNS cache if you have recently updated a DNS Record

Similarly the NetScaler itself is configured with the STA details.