Link Namespace fails: invalid_request "missing required code challenge

Subject: Link Namespace fails: invalid_request "missing required code challenge" 

   Dev Hub org ID: <xyz>. Namespace org ID: <abc>. Namespace: Nativesign. 

   Namespace Registries > Link Namespace popup returns error=invalid_request&error_description=missing required code challenge. 

   The connected app "SalesforceDX Namespace Registry" has PKCE checked and locked ("contact Support"). Org-level PKCE is OFF. How can I link my namespace?