Problem with SharePoint 2007 upgrade to 2010 - "One or More FieldTypes Are Not Installed Properly"

Recently I came across a problem when upgrading from SharePoint 2007 to SharePoint 2010.


I got a cryptic error telling me that one or more of SharePoint's field types had been installed incorrectly.

Looking at the Publishing Site "Pages" document library was in vain, none of the columns offered any obvious answer. After much toil and labor I found out that http://sitepath/Relationships%20List/allitems.aspx was to blame. The GroupID site column component, which was formerly a GroupGUID (in 2007), was a GUID no longer. So I entered site settings, switched off the Publishing Infrastructure site service, deleted the Relationships list, reenabled the Publishing Infrastructure and voila, everything worked.

Thanks to the peeps at:
http://www.go4answers.com/Example/error-starting-sharepoint-2010-beta-2-44876.aspx