I would try to take care of it at YYZ. AC may be able to handle check-in for both flights for you, but if not you can either talk to UA there, OR, when you get to ORD, there are Mr. Chickens at both the airside customer service areas and in some of the RCCs that you will be able to use.

Rather than being SSSS, it may be that Mr. Chicken Online is just barfing, as usual. The past few times it refused to check me in online, Mr. Chicken at the airport was able to do so just fine, without an SSSS or anything. Just his little bird brain, I guess.