I am having the exact same problem trying to check in for my parents for a flights to Montreal tomorrow. Have you figured out a solution? I also called Air Canada and the guy I spoke with was very vague and just kept presuming we had entered the info for the ETA incorrectly. This can't be the case as my Dad flew with his current ETA to Canada last year with no problems.

Have tried to access to Advanced Passenger Information System but can't update any details or see what details have been input as we are within 24hrs of the flight leaving.