Try importing that project into a new project that is working and see if it behaves.
If it works or if it doesn't, please file a bug to help make the next release less buggy.
Shame on me for jumping the gun on the OS update. But don't doesn't Adobe have the OS beta plenty of time before the release?
You would think, but this is pretty typical of Adobe. They tend to not officially support a new OS for a month or two after it's released. Sierra is very explicitly left off of the list of supporting operating systems for all versions of After Effects, so we have been warned.
There are a number of other big bugs with Sierra including issues with renaming layers, creating expressions, and scrolling in the composition panel using wireless Apple trackpads/magic mice.
The good news is that, if you look at the history of AE's releases, there is usually a somewhat significant update in October or November. If that happens again this year, I would expect it to support Sierra. So you can roll back now and avoid all of the issues or wait and hope the tradition continues.
Interesting. When I drop the project into a new one, I don't get the memory error. But nothing loads - just the checker background. I can play and everything. Sound works. Then when I clicked into a composition, AE crashed.
Unfortunately I can't roll back because I don't use Time Machine for backups. I guess I'll remember to do that next time. Unless you know of a better process. (I would love to roll back because my Corsair keyboard isn't working properly either.)
Excluding an OS that didn't exist at the time from the supported OS list isn't really a warning. Of course they aren't going to have it on the list. Sure, as a general practice, we shouldn't be upgrading our OS immediately, but they could do more to keep pleb users informed. And in turn, it would remind the more intermediate users from making dumb mistakes like this.
Maybe the dude who updates Adobe Reader every god damn day should be in charge of the software that actually matters. I honestly shouldn't have to pay a monthly fee for the time that their software isn't up to date.
Maybe the dude who updates Adobe Reader every god damn day should be in charge of the software that actually matters.