Named Credential Authentication Fails with Error " ErrorCode=Existing_link"

When a Named Principal is authenticated using a Salesforce Auth Provider (Salesforce-to-Salesforce OAuth flow), the OAuth login on the source org succeeds and is reflected in the Login History. However, the Named Credential Authentication Status remains permanently in "Not Configured" and the following error is displayed on every authentication attempt:

We can't log you in because of an authentication error. ErrorCode=Existing_link

The issue occurs because the target org's Data Storage is at or exceeding its limit. When Salesforce attempts to write the authentication token record post-OAuth, the write fails due to insufficient storage. The record is never committed, causing the Existing_link error on every subsequent attempt.

Root Cause
Target org Data Storage is at or exceeding its allocated limit. The authentication token record that stores the Named Principal credentials cannot be written due to insufficient storage space.

Note: If Data Storage is within the allocated limit and ErrorCode=Existing_link continues to occur after authentication, this may indicate a different underlying cause. Please contact Salesforce Support for further investigation.