ADFS Sign-in Only for Some Users

Question

I recently set up an ADFS farm, and I am testing a few user accounts to see if they can sign into /adfs/ls/IdpInitiatedSignon.aspx. Some accounts work fine; others, however, exhibit the following behavior:

• Signing in with a wrong username/password results in an error message indicating the username/password is incorrect.
• Signing in with correct username/password results in a page refresh and the sign-in form is displayed. There is no error message; simply a page refresh and the sign-in form.

In the Security event log, there are three events related to the sign-in. They are as follows (in arrival order):

Event 4648 - A logon was attempted using explicit credentials.
Event 4624 - An account was successfully logged on.
Event 4625 - An account failed to log on (Failure reason: Unknown user name or bad password)

I'm at a loss as to why this is happening for only certain users. Any help you can provide is appreciated.