This issue has been reported by bluebird2 here: http://wordpress.org/support/topic/258314 and I will continue in a new thread to clarify.
In some WordPress installs, when activating the widget, the WordPress gallery does not show. Until now I have not reproduced this issue, and today I did in a site where I newly installed SidePosts.
This is a estrange behavior, and by now I don't know where is the problem as it only is reproduced in one of the five sites I tested it on it. And all sites have the same configuration.
Opened a bug in the tracker and will debug this until solved.
This bug can be followed here: http://alkivia.org/tracker/view.php?id=52