BuddyPress.org

Sounds to me like a PHP error or a memory error.

make a file called php.ini, put it in your wp-admin directory, and inside that file it put…

memory=40MB

upload_max_filesize=10M;

post_max_size=20M;

This will take care of any memory issues your host may be having…

In the .htaccess file at your root directory of your WordPressMU install, put this somewhere near the top…

AddType x-mapp-php5 .php

AddHandler x-mapp-php5 .php

This will take care of PHP issues and make sure your host is giving you PHP5.

Try these out and report back.