Re: wp_sitemeta table in db crashes often
I do not know if the NextGen Gallery plugin modifies the wp_sitemeta table. I suggest that you disable the plugin and see if the issue goes away.
If you are running a newer version of WPMU than is currently supported by NextGen Gallery, you could have issues.
Also, I do know that there are some issues running BP with the NextGen Gallery plugin–with avatar cropping. It’s possible that is could be causing other issues as well. Contact the developers of that plugin and ask for their help and whether or not they plan to support BuddyPress.