You've purchased the SimpleRisk Custom Authentication Extra and enabled it.  You added a new user, but after that user enters their username and password in order to log in, all you see is a blank screen like this:

If that is the issue which you're experiencing, the user you are trying to log in as has likely accidentally been configured to use Duo as a second factor for authentication, but your instance has not been properly configured for Duo MFA.  If this is the case, you have two options to fix the issue:

Option 1: Remove Duo MFA from the Affected User

Go to Configure -> User Management.  Scroll down to the section labeled "View Details for User".  Select the name of the user that you are having an issue with.  Scroll down to the bottom of the "Update an Existing User" page and find the section for "Multi-Factor Authentication".  Change the selected value from "Duo Security" to "None" and click "Update" to save the user's configuration.

Option 2: Configure Duo MFA

Go to Configure -> Extras.  Select "Yes" in the Enabled column for the Custom Authentication Extra.  Scroll down to the "Duo Security" section and enter the IKEY, SKEY, and HOST values for your Duo account.  Click "Submit" to save your Custom Authentication Extra configuration.