BuddyPress.org

I haven’t seen this on my dev site, and I don’t think we’ve had reports of this, so… as it’s staging, please can you switch the theme temporarily to a default WordPress theme, like twenty-twelve (etc) — and then visit this same URL again and see if the warning is still being printed or not?

I suspect it’s something in your theme, maybe one of the templates, or some customisation hooked into the members loop, but switching the theme is an easy way to start figuring out possible causes.