Press the caps lock key to turn caps lock on. Close the dialog. Fix the problem. Turn caps lock off.
Thanks for your suggestion Rick. Unfortunately, this didn't work... any other ideas?
The problem is easily reproduced, and I can't find any way out of it. It's worth posting as a bug to the AE team, because really the software should be able to escape from the infinite loop it creates.
In the short term, it appears to me you have no option but to force quit. Hopefully you are using regular auto-saves so you don't lose more than a few minutes work.
In the future, I'd recommend you simply get diligent about not editing the scale of any camera parented layers. Ultimately this is a user error, after all.
Thanks Andrew. As I'm just new to learning After Effects and finding my way around, I think these errors come with the territory! But this is one I won't be making again haha.
Thanks again for your reply!