SAML inline Hooks - how to troubleshoot Bad Request?

Hello,

I am trying to add a SAML Inline Hook.
I have a SAML Application configured, and a user assigned to this application.

I create a hook pointing a node express app, responding a very simple ‘add’ command, just to test.
I use the “Preview” tab of the SAML Inline Hook.
It generates the request for the user and a the SAML application I provisioned.

When I click on the “Preview Response” button, I simply get a “Bad Request” error.
When I go into the System Logs, I see there is an error, with no further details.

When I copy the generated request from the Preview section into Postman, and I use the URL I configured in the inline hook, I get my json payload.

My question is: how can I troubleshoot this?

Thanks!