I got this error after upgrading from 4.5.4 to 4.5.5 and found that I might have accidentally navigated to my web site from an external browser after I copied the files over to the new site. Thus I think it corrupted the upgrade procedure.

To fix the issue I just navigated to the following url on the local machine <siteName>/Install/Install.aspx?mode=upgrade

Upgrade process went ahead just fine.

Got this info from Mitchell Sellers entry but I didn't bother about setting the autoUpgrade to false as they suggest. Didn't need to uncomment the Authentication either as suggested in another DotNetNuke forum article.

FYI