Signals Notebook: Warning message "Attribute <attribute name> cannot be modified" when attempting to copy an experiment

Description

Users are unable to copy their own experiments with a warning "Attribute <attribute name> cannot be modified" shown by the network log and warning "You do not have the necessary privileges to create this Experiment" in the UI  

Cause

The experiment contains a read-only attribute field with a value no longer associated with the user's system group. This can happen because the value was removed from the attribute list, or the group association for the value was removed.

Solution

Administrators should a) add the value to the list once more, b) add the group association to the value again, c) ask users to no longer copy from experiments where the value was used.