I am flying tomorrow from LAX to EWR. I fly this route about 10 times each way a year. When I tried to check in today I was told, "We are unable to complete the check-in process at this time. Please proceed to the airport for assistance." Why?

I tried online with Firefox and Chrome and with the United app on my phone. I am a US-born citizen with Global Entry and this is a domestic flight anyway. Makes no sense.