This started when I changed "show last user" on sign-in screen to disabled.

I have two user accounts: Randy1 is a regular user; Randy2 is an admin. After I made the change above, I could still login with Randy1, but Randy2 always gave me an "invalid name or password." At first I thought I was really screwed. No usable account with admin privileges! However, it is weirder than that.

In Randy1, let's say I attempt something requiring admin privileges. I get a popup with Randy2 in it and a prompt to enter my password for Randy2. This still works. That tells me both the account and password are good. There is just an oddity with the logon name.

I go into Control Panel, User Accounts. I click Manage Another Account. Again, I am prompted for the password for Randy2 to get admin privileges. This works. As a precaution, I add another admin account, Randy3.

I reboot. I can now login normally with Randy1 or Randy3, but still not with Randy2. When logged on as Randy3, I go back to User Accounts and try to change the name of Randy2 to Randy2, thinking maybe there is some hidden character messing up the logon of Randy2 and I might be retyping the name now cleanly. I am told that "Randy2" already exists on the system.

Finally, I give up and restore the default "show last user at logon." Now I can login any of the three users.

Anyone have an idea what might be happening here?