Hi, Guys
Allen here from WP e-commerce team.
Here is a fix to that “WP-Error” error go to wordpress admin page, go to options->writing select a Default post category and save the change.
We are still trying to find that anything in our code triggers the bug, but for the two site i have fixed, both of them have like more than 20 plugins,it is really hard to tell which plugin causes the error.