I sent you a message 2 days ago. Basically, I have managed to replicate the kind of error you have: Pages are not loading within application but loading properly when you open in in normal browser.
I have blocked the application when the firewall asked about his and the result was exactly the behavior you mentioned. This was also true when I closed BitDefender or disabled it's firewall. The solution was to put fastIBL in the firewall's allowed list.
This is something that was reported by some other fastIBL users too. It worked ok until a moment when is starting to behave as you mention. Unfortunately, I still work on discovering what might be the problem. This process is complicated by the fact that I did not managed to replicate this problem on one of my computers so that I can study it properly. Basically, the same app. version and the same db were functioning ok if moved to a different computer.
One solution found so far was to use a different fastIBl version. Do you have the latest version taken from the server? Pls. try this first and let me know if it starts working.