Migrated the public website from 2007 to 2010.
I’ll get an error when accessing the Site Permission page (/_layouts/user.aspx): ContentPlaceHolder "PlaceHolderHorizontalNav" missing.
We had a custom master page attached to the Application Pages. This masterpage didn’t had that placeholder in SP 2007. I’m now just using the "v4.master" Masterpage for the application pages… but you can just easly add that placeholder via SharePoint Designer 2010 to your own custom masterpage…