Same thing happened to me. Talked to united.com tech support...which redirected me to united.com "un" check-in line...which checked me out...
Apparently you have to use MSFT Internet Explorer in order to have the documents print correctly...
But beware, my seat assignments were given up when they checked me out and they put me in the back of the plane....
After calling the reservations line to have them put me back in my seats, they told me that I already was...lady in HNL Call center talked to her supervisor and had to call the airport directly to have them reassign us to our originally assigned seats. The CSR in HNL was great as I told her I had to take another call. She continued working on the problem and called me back to ensure that everything was fixed.