The After Effects CC 2015 (13.6.1) bug-fix update is now available. This update fixes multiple bugs, including a bug that caused After Effects to not update the preview image in the Composition panel, which happened most commonly after using Undo.
You can install the update through the Creative Cloud desktop application, or you can check for new updates from within any Adobe application by choosing Help > Updates.
For details of what was added, changed, and fixed in After Effects CC 2015 (13.6), see this page. For details of all the other updates for Adobe professional video and audio applications, see this page.
The After Effects team is continuing to investigate other bugs for future updates. Refer to this article for current information about known issues.
The After Effects CC 2015 (13.6.1) bug-fix update addresses these significant bugs: