In case this helps anyone, I stopped being able to book using the website and was getting errors regarding "incomplete stopover details" (even on a simple one way itinerary involving no stopevers) when hitting the final "accept and pay" button in the process. Finally decided to download the app and try there, where I got an error message that made sense. I had been trying to pay using a TD card that I had applied for a few months ago via Air Canada and it had been automatically added to my profile. It had worked fine for a few months but I guess eventually you stop being able to use the card without adding it "properly". Re-added the TD card to my profile and I can book again.