@tbel then my hypothesis is that such “Action Cannot Be Completed” appears when the app tries to update the property of the user to which it doesn’t have an access.
I.e. if you have Current User somewhere and try to update a Full Name, and your access rights are set to default ones in the User Permissions (only record owner can change), you will get this result.
I’ve had this on my several projects… Permissions change helped.
Best,
Victor.