With a valid CC subscription, you can install CS6, CC, CC 2014, CC 2015, CC 2015.3, and CC 2017 via the Creative Cloud app. They can all be installed alongside each other with no problems. This page shows how (hint: there are TWO places where it says "previous versions" and the one that looks right, isn't. )
That being said, if you're not in the middle of a project, I'd like for us to be able to help you get it working. AE has so many nice changes in CC 2017 that I'd hate for you to miss out. Can you open AE and start a new project without issues?
Everybody who is having this issue, please install the latest update that came out last night: After Effects CC (November 2017) Bug-Fix Update Is Now Available
This update is 14.0.1 and has bug fixes for several disturbing issues - especially for Mac users. It does not fix all the bugs that are new with the CC 2017 release (Known issues in After Effects CC 2017), but it does fix some issues that have been plaguing After Effects since the first parts of the new architecture showed up in the CC 2015 release. So see if it fixes this issue for you.
If you can't see the update yet, you may need to wait a while until you see it as it can take up to 24 hours to populate an update across all the data centers. You may need to log out, relaunch the Creative Cloud app, and long in again.