If a user tries to change a parent on a portfolio item(E) and if the existing parent(T1) on E is in a project to which the user does not have edit access, then the user gets the following error. Keep in mind, the user has edit access to E, and wants to update the parent to T2.
(To put it another way, user has edit access to E, edit access to T2, but no just view access to T1. E has parent as T1 currently. User tries to change T1 to T2 and gets the following error). Error setting value for a field [Parent ]: Not authorized to update the current value.
This functionality is working as designed. The reason for this is that when a Parent is removed or added to an artifact the Revision History and any statistics are changed for both the Parent and the child Portfolio Item. Essentially, the Parent that is being removed from the Portfolio Item is being updated/changed too. So if a user only has viewer access to a project, the ability to change/edit anything in that project in any way is not allowed.