Resolve Enablement Site (myTrailhead) Access Issues after Org Migration or Instance Refresh

If you have an enablement site (formerly a myTrailhead subdomain) and the org associated with the site has recently completed an org migration or instance refresh, users can have trouble logging in to the site and accessing content. This issue occurs because the org migration or instance refresh doesn’t propagate the relationship between your permission sets for enablement site users and specific content collections that users can access.
For example, if your organization recently completed an org migration and your enablement site users report they can no longer log in or see their assigned content, this article describes how to restore access.