To retain Millimetres as the default application-level Unit, try this:
(1) Quit Illustrator.
(2) Go to <user>/Library/Preferences/Adobe Illustrator 22 Settings/en_US and delete "Adobe Illustrator Prefs" file.
(3) Go to <user>/Library/Preferences/ and delete "com.adobe.Illustrator.plist" file (this is a legacy preferences file from older versions).
(4) Illustrator should then retain any application-level preferences you choose.
Regrettably, this does NOT fix the Units within the New Document dialogue box — all presets/templates keep defaulting to Points, despite Millimetres being the application-level preference. Adobe needs to fix this! :-(
To retain Millimetres as the default application-level Unit, try this:
(1) Quit Illustrator.
(2) Go to <user>/Library/Preferences/Adobe Illustrator 22 Settings/en_US and delete "Adobe Illustrator Prefs" file.
(3) Go to <user>/Library/Preferences/ and delete "com.adobe.Illustrator.plist" file (this is a legacy preferences file from older versions).
(4) Illustrator should then retain any application-level preferences you choose.
Regrettably, this does NOT fix the Units within the New Document dialogue box — all presets/templates keep defaulting to Points, despite Millimetres being the application-level preference. Adobe needs to fix this! :-(