Hi Alex,
Looks like some kind of permission related error in which you are not allowed to write content. This is not caused by Boxer but is more like a configuration setting in your environment. Make sure the boxer/assets/css directory is chmod'd to 777 like it says here.
Also, the boxes are not gone. WP Boxer just uses another post type than the previous versions. You can easily convert them using the Upgrade From Older Version option in Settings | WP Boxer.
Let me know if this works!