BuddyPress.org

== it breaks smooth slider plugin and multi level Navigation plugin ==
It would help if you were more specific about which smooth slider plugin and which multilevel navigation plugin you are referring to. Are those the names of the plugins or the generic terms for such kinds of plugin?

There have been many issues of WP plugins/custom theme incompatibility with WP 3.1 due to many changes in core. Btw, what theme are you using? WP 3.0.5 upgrade to WP 3.1 has broken “some stuff” along the way. https://wordpress.org/support/topic/troubleshooting-wordpress-31-master-list?replies=11

On each plugin’s homepage in WP repo, look to the lower portion of the right sidebar which links to forum topics where people give feedback – report problems.

– Re Smooth slider – https://wordpress.org/support/topic/plugin-smooth-slider-needs-upgrading-for-wp-31?replies=2

– As for multilevel navigation plugin – unfortunately, the plugin developer doesn’t seem to provide support in WP repo or his website, and it supports WP version up to WP 3.0.5 only, while BP 1.2.8 is compatible with WP 3.1. In other words, this is not a BuddyPress issue at all. Suggest that you look for alternative plugin