What does it mean if I get the error message "We are unable to complete the check-in process at this time. Please proceed to the airport for assistance"?

Premier line rep told me "there might be a glitch, we've been getting some calls tonight," but was unable to check me in. Web support rep said the same thing.

This is for 9:15pm EWR-ORD tomorrow (last flight of the day). Should I anticipate trouble, and allow for extra time at the airport?

(PS: first flight on United in more than a year. In the meantime, ~20 trouble-free segments on a competitor.)