Dear Todd Thornton
Have you tried deleting entry 20? Is it in anyway strange?
Can you put the glossary somewhere on the net so that we can try it?
Is there a difference in encoding between the two systems? 1.6 supports UTF-8 databases. If there is an encoding problem then you could try unzipping the backup, converting the ecoding of the XML inside and rezipping.
How about creating a test 1.5 moodle with the glossary and upgrading that installation to moodle 1.6. The upgrade routine may do more checks than the backups. I know that the upgrade has in the past sometimes checked for known database issues.
If the worst came to the worst, there is probably a way of reformatting the database XML (again inside the backup zip) into a blackboard style glossary and importing it as a flat file, somewhere on the forums. But you would loose keywords and other information.