I'm getting the same error for the first time ever!  Installed V4, the Morphs++ pack and Domino last night.  Created a scene, hit render, and after about 5 hours (just when the render was about to finish) P6 crapped out on me and completely froze up.  I had to Ctrl-Alt-Delete out.

I had saved my scene, so I thought, okay no problem...I'll just try again with different render settings.  So today I open up Poser, try to open my scene and I get that XML error.  I've tried deleting LibraryPrefs.xml and Poser UI Prefs_1024x768.xml and that doesn't help at all.

Whenever I try to open my V4 scene, my work window and Parameter Dials disappear.  I try to open the file again or close out of Poser and I get a message to save my file...so it's like it's there but it's not.  When I try to open an older scene file the Parameter Dials disappear but the work window is there, but completely blank!  What the hell??????

"You don't know what we can see
Why don't you tell your dreams to me
Fantasy will set you free." - Steppenwolf