Hi guys,
with further testing I have ascertained that the error happens when the examined arraylist is in a gameobject along with others arraylists. Regardless the fact that a unique reference is used for the array, it seems that the actions fail to complete.
I have isolated the array in it's own gameobjects and the action works fine.
My arraylists are not created through "create array list" actions. They pre-exist and are filled with data during runtime. Thank you all for your feedback.
(I can't send the scene Jeanfabre, because it's big project and there are security issues. Thank you for your support.)
It would be great if this action could give a result of the duplicate values in a string.....