I got "WP Post Styling Library Update Failed" whenever I tried to add a custom style.
I checked the code and found that it seems they database wasn't created somehow. So I created a new one according to the prefix, i.e main_5_post_styling_library, and it worked.
I'm using WordPress 3.5.1 with multiple sites. I haven't tried on my other sub-domains, but I suppose it would be the same.