When users use the login button, they only see a blank page

Some users do not automatically have access to the named credential that was created, this is not included in the Filenize User or Admin permission set. To resolve this, you need to clone the Filenize User or Admin permission set, edit it, open Named Credential Access, and add your named credential to the Enabled Named Credentials column. Or provide access to those users with another new permission set.

 Users don’t see Authentication Settings for External Users under their personal settings

Salesforce hides the menu item if the user does not have access to any named credential that exists in the org. This can be resolved using the steps described in 10.1 

As an admin i don’t see my sites or subsites when configuring my SharePoint instance

This can be due to a couple of reasons :