Users who are running macOS 10.11 (El Capitan) with Adobe Illustrator with FontAgent 8 and auto-activation enabled will likely see Adobe Illustrator crash when opening an existing document that requires fonts to be auto-activated.

This issue is a bug between Illustrator and macOS 10.11 (El Capitan). It only effects macOS 10.11. We recommend that you upgrade to macOS 10.12 (Sierra) or later.

If upgrading to macOS 10.12 or later is not an option, you can disable auto-activation for Adobe Illustrator by removing the auto-activation plug-in from this location: /Applications/Adobe Illustrator/Plug-ins/FontAgent.aip
This will prevent Adobe Illustrator from crashing when opening documents, but you will not be able to auto-activate fonts.