I had the same problem a couple of weeks ago.  Poser9, XP.  I had used a texture set that used a python (don't want to say which texture, because I don't want to attach any blame to the creator).  Thought something about that python set up might have screwed up something in the P9 ini file.  Anyway, since i still have P7 and P8 on my computer (just in case :D), I wanted to see if this "bug" affected P8.  Nope.  The library window opened fine.  So, I again went to P9 (insanity:  doing something over and over again, expecting a different, desired outcome).  And, surprise of surprises, the library opened in P9.  Not crazy after all.

Perhaps something in P8's working library "awakened" P9's library...haven't a clue.  So, if you still have P8 on your machines, give it a shot...

All I have to say is, that for so many to have this problem, it could  be attached to some type of updated program that Poser uses...??