The Openbox update did not go well for me, either. After the update, none of the menus were available. No way to logout, reboot, etc., from a menu. Did a Ctrl-Backspace to restart X. After logging in, I had ... nothing but a black screen. No autostarted apps, no wallpaper, no tint2 panel. But, I did have menus again. On the offhand chance a reboot would set things right, I tried that. Same scenario.
I restarted X and logged into an IceWM session. After reading this thread, I opened Synaptic and installed libopenbox2. Logged out of the IceWM session and logged into an Openbox session. Much better. Things are almost back to normal. Had to reset the wallpaper (no biggie). Settings were looking at my home directory for wallpaper instead of /usr/share/wallpapers. I still have to figure out what happened to my tint2 panel borders, colors, etc.
I can assure you that this installation is 100% PCLinuxOS, pure and clean, no outside additions. IMHO, libopenbox2 should be added as a prerequisite or dependency.