This is the theme author’s answer:
As I suspected, there is a nesting issue. When used in a sidebar, the loop is finished, so there is no nesting problem. But [weaver_show_posts] already follows all WP rules and standards for resetting itself, but it is possible the next gen short code is relying on some global value that applies to the page or post the [weaver_show_posts] is on, rather than the post displayed by [weaver_show_posts]. This would be a bug in the nextgen code.