Windows SharePoint Services 3.0 was unable to backup because of a field type 'HTML' error. To resolve this, you have to remove the “rogue” references to the content type. It's not a pretty process, but it did work. You may have to write up some code to identify which lists are referencing the bad content type if you have a huge site. Just make sure you back everything up before mucking around in the SharePoint database.

This was the original error I had:

Field type HTML is not installed properly. Go to the list settings page to delete this field.    at Microsoft.SharePoint.SPFieldCollection.CreateSPField(Int32 index)
at Microsoft.SharePoint.SPFieldCollection.EnsureSPField(Int32 index)
at Microsoft.SharePoint.SPFieldCollection.get_Item(Int32 iIndex)
at Microsoft.SharePoint.SPFieldCollection.ItemAtIndex(Int32 iIndex)
at Microsoft.SharePoint.SPBaseCollection.SPEnumerator.System.Collections.IEnumerator.get_Current()
at Microsoft.SharePoint.ApplicationPages.FieldListRenderer.OnPreRender(EventArgs e)
at System.Web.UI.Control.PreRenderRecursiveInternal()
at System.Web.UI.Control.PreRenderRecursiveInternal()
at System.Web.UI.Control.PreRenderRecursiveInternal()
at System.Web.UI.Control.PreRenderRecursiveInternal()
at System.Web.UI.Control.PreRenderRecursiveInternal()
at System.Web.UI.Control.PreRenderRecursiveInternal()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)