This is a quick response but I think it should show the error:
Created a business object: Contact
with a few Attributes: Name / Phone / Notes
Add a new Attribute: LinkUsers
Type: RegularUser
Relation to RegularUser: PEER
Matching Attribute in RegularUser: CREATE LinkContact
On the CONTACT FORM be sure you have a TABLE OF ITEMS for the LinkUser attribute. So that when you bring up a contact form it has a list of users.
Create a NEW Contact
Click on ADD NEW to create a Linked Login
Type a Login Name
Type a Password
Click Created (FYI: The login record did get created in the database and shows without error)
Click Create
List Contacts via some query
Click on the contact you created above
Click on the ADD NEW to create another Linked Login
Type a Login Name
Type a Password
Click Created (FYI: The login record did get created in the database and shows without error)
Click Save (popup record change ERROR Shows)