Hi, @James Hamil thanks for the reply, we have upwards of 2000 users logging in fine with this flow from a variety of home and office networks and different devices. It's just that we have 1 user reporting that when they attempt to login from their office network, they can't.
On a troubleshooting call, we noticed that the they get ERR_CONNNECTION_CLOSED when the login page tries to load in the custom UI page and we have a follow up call with them scheduled where we'll be able to gather more information from their browser.
It doesn't seem like a general CORS or script execution thing as we only have this one report of an issue and only from this user's office network.