Heres an explanation on the problem when encounterd in Poser 5, I saved it from a site back in the days and I dont know who is the author:

 

XML parse errors can occur if you have a library (either created in Poser
5, or an old Poser 4 library) that has one of the following characters in
the name:
quote (")
apostrophe (')
ampersand (&)
less than (<)
greater than (>)

(These may not cause a problem, but may be problem characters)
back slash ()
slash (/)

To solve the problem, you must rename the offending folder or file. Then you must fix the reference in the LibraryPrefs.xml file