BuddyPress.org

So this was dumb. The issues was because of a trailing slash after `activate`

Compare:
`example.com/activate/?key=d2373a345c3bbf9a62dd475e61cfbff9`
`example.com/activate?key=d2373a345c3bbf9a62dd475e61cfbff9`

Seems that WP assumes apache and creates .htaccess that our nginx ingnores.

Thanks for your help @davidtcarson