When this happens, make sure you are on a compatible version of Citrix Receiver.
In some cases, resetting the Citrix Receiver has resolved this issue. To do this, follow these steps:
1) Right-click on the Citrix Receiver icon in the hidden icons tray
2) Select "Advanced Preferences"
3) Select "Reset Receiver" from the screen that appears.
4) After selecting this, you should be able to log into FIMS through HostNet.