Error with upgraded publishing site

Today I tried creating a new Welcome page on a site that has the publishing feature activated. The site was upgraded from 2007 to 2010 long ago with no hiccups. Suddenly just trying to create a new page threw a runtime error that required digging through the SharePoint Log with a correlation ID to find:

The site is not valid. The 'Pages' document library is missing.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: Microsoft.SharePoint.Publishing.InvalidPublishingWebException: The site is not valid. The 'Pages' document library is missing.

The Pages library is certainly on the site. Deactivating and re-activating the publishing features from the site collection and site features lists didn't seem to have any impact on the error. SharePoint engineer to the rescue: Stefan Goßner has your fix!

http://blogs.technet.com/b/stefan_gossner/archive/2011/09/15/common-error-the-site-is-not-valid-the-pages-document-library-is-missing-error.aspx