RE: CRM 2016 session expiring and keep on asking for credentials after entering also
The reason for the behaviour is that your authentication security token has expired.
The lifetime of a default security token for a claims-based authentication is 60 minutes. You can extend the security token lifetime as shown here:
technet.microsoft.com/.../gg188586(v=crm.6).aspx
The authentication prompt that you get after the token expires is for Windows Authentication instead of the ADFS login prompt - which is why trying to enter the credentials in the popup doesn't work. (Not sure why the browser does that.)
The easiest way to get around this popup is to close and re-open the browser and navigate back to the CRM URL. You will then get the ADFS login prompt.
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