Either you misunderstood me or I am misunderstanding you.

The error is not when I am doing the conversion. It is when I am "adding" the book. e.g. I go to Add Book and click the top menu item. Then pick the html source and hit Open. I get a dialog box that says "Adding..." and it is blank except for "Add...". After a few minutes, it stops with an error message that reads: ERROR: Adding failed: The add books process seems to have hung. Try restarting calibre and adding the books in smaller increments, until you find the problem book.

The html I am trying to load is pointed to by my original post. I'm happy to try and help debug this.