If either ADFS (Active Directory Federation Services) or ID (DigaSystem Identity Server) is enabled in DPE web.config an auto logon with windows credentials is performed.

To be able to optionally log in with other credentials you have to use the query parameter nosso in the logon URL, e.g. http://dpe-vm2016/DpeWebApplication/Logon.aspx?nosso.

If ADFS and ID are enabled auto-logon uses ADFS.