BuddyPress.org

@marximusmg
There must be something different between the two sites if they are identical yet that plugin does not work on one of them. You may want to try to retrace your steps to identify what the difference is. Any different plugins, custom functions, etc.

@marximusmg
The only place I know to contact the author is on the plugins page support forum.
On the site that you are using it successfully are you using a different theme? If so you may want to try activating twenty twelve or bp-default to see if there is something in the theme that may be causing the issue.

@marximusmg
Are you aware that the unconfirmed plugin is for multisite? Pending activations has not been updated to work with anything past BP 1.6.4 so maybe you could contact the author on the plugins support page and maybe he will update for BP 1.7+
Good-luck!