I also got that message after having a printer error. I couldn't get my printer to work and I didn't want the check-in process to complete so I could do it later after I resolved the issue. I did not confirm check-in and I hit the back button, where it gave me that same message. But when I logged out and logged back in, it had completed the check-in process anyway and allowed me to reprint the boarding passes. Prob not the same issue as yours, but in any case this seems to be another common united.com glitch unrelated to SSSS.