Update: New version v2.2.2 (for macOS only) addresses this compatibility. Details posted here.
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Recent updates to Illustrator specifically version 25.1 onwards have resulted in a need for plugin developers to patch their Mac builds to include a new method for building Mac OS based plugins. This is impacting opening .aix files on a Mac environment. The latest version of ArcGIS Maps for Adobe Creative Cloud plugin (2.2.1) is compatible with older version of Illustrator version 25.0.1. If you have recently updated Illustrator or have installed a fresh copy and you receive an error when opening .aix files on your Mac OS with the latest version of Maps for Creative Cloud plugin then we recommend rolling back Illustrator to a previous version to continue working. Rolling back Illustrator to version 25.0.1 or alternative installing an older version such as version 24.3 side-by-side, will allow you to continue working with .aix files. (Windows users are not impacted by these Illustrator updates).
Steps for rolling back Illustrator -
1. In your creative cloud app click the 3 dots next to Adobe Illustrator, expand the install menu and go to other versions
2. From the list of 'Older versions' list, Install version 25.0.1 or 24.3
Note: If installing 24.3, you will need to reinstall the plugin.
We are working on a patch to resolve this issue. In the meanwhile please try one of these suggested workarounds to continue working on .aix files on a Mac. A notification will be posted here when the update is publicly available.
Related thread:
Unable to open .aix file in Illustrator CC