BuddyPress.org

Are there other errors in your server’s log files?

Are you using a custom theme? If so, switch to the default BuddyPress themes and see if the problem goes away.

<buddypress>Do I have to install some jquery thing before using buddypress?</buddypress>

No. BuddyPress does not load the JQuery library but WPMU does. Currently, WPMU uses JQuery version 1.2.6.

Do you have any plugins other than Buddypress activated? If so, try disabling them and see what happens. There could be a JQuery version conflict somewhere.