Attached Link: http://www.curiouslabs.com/article/archive/86/?nf=1&#anc804

What version number is your copy of P5 ? This problem was was fixed in SR2.1 according to the CuriousLabs FAQ (see attached link), so you should get the latest service release.

If you are not using multiple runtimes, then the fix is simple, just delete RuntimeprefsLibraryPrefs.xml (make a copy first just in case), P5 will create a default file on next startup.

If you do have multiple runtimes, you may want to edit the copy created by Poser (look for a file with XMLError in it's name in the above directory), to avoid having to reconnect your other runtimes.
Proceed as follows, open file in text editor remove all lines that look like
Save a copy, then save as LibraryPrefs.xml

Until you update to a service release that fixes this problem you will get this error again if you exit poser with a library directory selected that contains an unescaped XML character.