I ran into the same problem installing Poser 8, but what probably caused my problem was that I didn't have the Poser 8 content or the legacy content downloaded yet, and I did a custom installation with just the Poser 8 content selected.

I solved it by reinstalling again with the compact installation feature (I never use the base Poser content anyway). That seemed to do the trick for me at least. I don't know if it'll help anyone else though.