Modify an existing app written in C# on .NET

We have an existing web app written in C# on .NET which can be seen on https://openscapecloud.net/ - The login no longer works and the developer who initially built it is no longer reachable.

The login currently uses the auth0.com service. There are 2 possible pathways to this:

1) Debug the auth0 problem and fix it OR
2) Implement a standard native login system as part of the app itself so it no longer relies on this external service to authenticate users.

It is hosted on Microsoft Azure. We can provide access to the source files on GitHub as well as full access to the hosting environment on Microsoft Azure.