Can you provide a copy of the error that you are receiving?
If you could also provide any other implementation details that would also be helpful in diagnosing. Such as what URL you are attempting to access?
Also, when you say normal authentication flows can you provide a bit more detail? One of the wonderful things about FusionAuth is that it can handle a wide variety of Auth scenarios and setups. Getting more details on your particular implementation would be helpful.
Thanks,
Josh