I got it to work! Somehow there was a custom login page assigned, even though we wanted the default customer center login page. I went to Setup > Company > Preferences > General Preferences and scrolled down to the Customer Center Login Page field and cleared what was in there. Now the default login page is working!

Hello! I've tried clearing the cache, using different browsers, and different computers. We haven't rolled it out widely yet, but it's affected 2 users so far.