New feature was added in Sitecore 9.1 to authenticate users with the help of Sitecore Identity server. As a result when ever you access the /sitecore/login page , first the request is authenticated by Sitecore Identity and then Sitecore client page is navigated.
If you don’t want Identity server authentication, then Sitecore also supports on how to disable it.
The below are 2 simple steps to disable the Identity server.
Go to this config file App_Config\Sitecore\Owin.Authentication\Sitecore.Owin.Authentication.config change the property value to false.
Go to this config file and enable the file ( remove the example extension)
That’s it. Now it will works as normal authentication without Sitecore Identity server.