@vels,

Again I want to mention that the user activation has no relation to wpForo and to any wpForo Addon. Users are registered and activated in WordPress core. There is no "User Activation" function in the core WordPress, this is a new featured added by a new plugin, I don't know which plugin allows you to activate users, but you should contact to that plugin developers or to the WordPress support.

The only thing I can recommend is keeping wpForo and wpForo addons active and deactivating all other plugins (especially the security and user related plugins) then test it again.