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.