I and several family members have tried to check in from our reservations page (after logging in to United.com). When we hit the yellow Checkin button, the following obscure dialog box appears:

Sign in
https://integration.united.com
Username
Password

This dialog does not respond to the United FF#/Password combination.

The workaround is to checkin on the phone or directly at united.com (without logging in to united.com). Very annoying.