It is not the first time that I noticed there is something strange in the behavior of the preference "Additional Plug-in Folder" in the Plug-in & Scratch Disks section of Illustrator CC 2021 (25.1)
When I'm trying to change it, the preference is not saved. When I restart Illustrator, the option is enabled but instead of showing the path where the plug-in are loaded (this is the default behavior when it works), there is an empty string. My plug-ins are not loaded by Illustrator. It is not specific to my plug-ins. The plugins in the SDK are not loaded anymore.
I cannot develop and test my plug-in correctly. I cannot switch between the debug and release version of my plug-in by changing this preference. To use it, I need to make a symbolic link inside
/Applications/Adobe Illustrator 2021/Plug-ins.localized to my plug-in.
Moreover one my customer today reported me the same issue.
I don't know the exact conditions when this problem starts to happen. I have uninstalled all versions of Illustrator versions on my computer and delete the preferences. When I install again Adobe Illustrator CC 2021 (25.1), I can set only once the preference which seems to work and if I'm trying to change it again the preference will be broken. Clearing the preference does not work. The only solution is to uninstall and reinstall Illustrator again.
I noticed this behavior on macos but it seems to be present on Windows also
Has anyone experienced such a behavior and found a solution ?