Exchange Online email applications stopped signing in, or keep asking for passwords? Start here.

I have a client in this Tenant using O365 Business Apps, and It wont accept Modern Authentication.  It didnt have any of the registry keys that were mentioned that should have been put in place by the Apps, so I placed them in the registry.  I have attempted to repair the apps, completely uninstall the apps, and reinstall them.  I have deleted all profiles in Outlook and attempted to re-add, it still wanted to use Basic Auth.  I have added a host entry for their autodiscover record and tried to add the profile manually from the control panel, using manual settings, and again, basic auth.  I have temporarily re-enabled Basic Auth, but I'm running into this with multiple clients that we work with.