I have had this before without being able to find the real problem.
After doing some edits on the xml it starts giving me this error.
From Safari:
this page contains the following errors:
error on line 773 at column 3: Comment not terminated
<!-- place text hotspots here
<hotspot name="tex4" sty
Below is a rendering of the page up to the first error.
From Firefox:
XML Parsing Error: not well-formed
Location: file:///Volumes/IOMEGA-2.1TB/DNM-1864/1864-tour/data/tour.xml
Line Number 773, Column 5:<!-- place hotspots here-->
----^
There is nothing wrong the comment is terminated and if you remove the comment completely the error just jumps down to the next comment.
My backup which has exactly the same comments works without errors.
Hans