The message “An unexpected error has occurred. Please click on the Return to classic link to visit the classic content type gallery” may appear when accessing the Content type gallery in SharePoint Online as shown below.
This is because the current user, even if it has the SharePoint Admin role assigned, is not a site collection administrator of the content type hub site collection. To rectify this, the user(s) who need access to the content type gallery need adding to the Site Collection Administrators. This was pretty simple before as the classic link was available and the content type hub was listed in Active Sites within the SharePoint Online Portal but not anymore.
So the workaround is to following the steps below (ensure the user has the SharePoint Admin role assigned):
- Navigate to https://<tenant>.sharepoint.com/sites/contentTypehub (replacing <tenant> with your organisation name)
- Click the SharePoint cog and select Site permissions from the menu options
- Next click Advanced permissions settings
- From the ribbon on the users permissions screen click Site collection administrators
- Add in you user(s) and click OK
Or to access the Site Collection Administrators page directly navigate directly to https://<tenant>.sharepoint.com/sites/contentTypeHub/_layouts/15/mngsiteadmin.aspx (replace tenant with your organisation name).
You should now have access to the Content type gallery from the SharePoint Admin Portal.